High strength steel sheet having excellent formability and method for production thereof

ABSTRACT

A high strength steel sheet having (2-1) a base phase structure, the base phase structure being tempered martensite or tempered bainite and accounting for 50% or more in terms of a space factor relative to the whole structure, or the base phase structure comprising tempered martensite or tempered bainite which accounts for 15% or more in terms of a space factor relative to the whole structure and further comprising ferrite, the tempered martensite or the tempered bainite having a hardness which satisfies the relation of Vickers hardness (Hv)≧500[C]+30[Si]+3[Mn]+50 where [ ] represents the content (mass %) of each element, and (2-2) a second phase structure comprising retained austenite which accounts for 3 to 30% in terms of a space factor relative to the whole structure and optionally further comprising bainite and/or martensite, the retained austenite having a C concentration (CγR) of 0.8% or more.

TECHNICAL FIELD

[0001] The present invention relates to a high strength steel sheethaving excellent formability (stretch flange formability and totalelongation). More particularly, the present invention is concerned witha high strength steel sheet having both high strength of the order of500 to 1400 MPa and excellent formability in an ultra-high strengthregion, further, a high strength steel sheet also superior in fatiguecharacteristic, and further a high strength steel sheet also superior inbake hardening property [hardening property after baking finish, may bereferred to hereinafter also as “BH (Bake Hardening)” property] whichcan ensure a high strength by baking finish.

BACKGROUND ART

[0002] Steel sheets used after pressing in automobiles and industrialmachines are required to possess both high strength and high ductility,which requirement has been becoming more and more strong in recentyears.

[0003] Heretofore, as a steel sheet having both high strength and highductility there has been known a composite ferrite-martensite steelsheet [dual phase (DP) steel sheet] comprising ferrite as a base and alow temperature transformation structure contained therein whichstructure is constituted mainly by marternsite (see, for example, JP-ANo. 122820/1980). This steel sheet is not only superior in ductility butalso characteristic in that yield elongation does not appear due to alarge quantity of free dislocation introduced into a martensiteproducing region, and yield stress becomes lower, and that therefore ashape freezing characteristic in working is satisfactory. By makingcontrol to the aforesaid structure there is obtained a steel sheet highin tensile strength (TS) and superior in elongation (El) characteristic,but inferior in stretch flange formability [hole expanding property(local ductility)].

[0004] On the other hand, as a steel sheet superior in stretch flangeformability there is known a two-phase steel sheet of ferrite andbainite (see, for example, JP-A No. 145965/1982) . This steel sheet, incomparison with the above DP steel sheet, is superior not only instretch-flange formability but also in resistance-weldability(especially there is no softening of a heat affected zone) and infatigue characteristic. However, there is a problem that the steel sheetin question is inferior in elongation characteristic.

[0005] Further, there is known a retained austenite steel sheet whereinretained austenite (γ_(R)) is produced within the structure andundergoes induced transformation (strain induced transformation: TRIP)during deformation in working to improve ductility. For example, JP-ANo. 43425/1985 discloses a steel sheet which is high in strength andextremely superior in ductility and which is produced by controlling thestructure of a composite phase steel sheet into a structure having 10%or more of ferrite and 10% or more of γ_(R) in terms of volume fraction,with the balance being bainite or martensite or a mixture thereof. It isdescribed in the above unexamined publication that with such astructure, not only the strain induced transformation effect of γ_(R)but also high ductility is exhibited by soft ferrite, resulting inductility being ensured by ferrite and γ_(R) and strength ensured bybainite and martensite. However, also in the case of this steel sheet,like the foregoing DP steel, there has been a problem of stretch flangeformability being unsatisfactory.

[0006] In view of the above-mentioned problems, studies have been madefor providing a steel sheet superior in such formability as stretchflange formability (hole expanding property) while ensuring goodstrength-ductility balance based on γ_(R). JP-A No. 104947/1997discloses a steel sheet having a three-phase microstructure of ferrite,bainite and γ_(R) and with a ferrite occupancy rate/ferrite grain sizeratio and γ_(R) occupancy rate being controlled to predetermined ranges.This is based on the following knowledge: “An increase of γ_(R) bringsabout improvement of strength-ductility balance and of total elongationand the effect thereof is enhanced by microstructurization; further, asγ_(R) becomes finer, formability such as stretch flange formability isalso improved.” However, the improvement in stretch flange formabilityis low and it is keenly desired to provide a high strength steel sheetfurther superior in stretch flange formability.

[0007] Further, for the application of a high strength steel sheet toautomobile components, especially such structural members as automobilebody members and frames or suspension members such as suspensions andwheels, it is required for the steel sheet to be superior not only inthe foregoing elongation and stretch flange formabilitybut also infatigue characteristic [fatigue endurance ratio (fatigue strength/yieldstrength)]. Generally, low alloy TRIP steels involve the problem thattheir fatigue characteristics are deteriorated by martensite of a secondphase structure (martensite resulting from transformation of retainedaustenite).

[0008] Further, in applying a high strength steel sheet to suspensionmembers of an automobile as referred to above, it is required for thesteel sheet to be superior in bake hardening property (BH property) . Asto this BH property, it is presumed that, by baking finish afterworking, C (solid solution C) dissolved supersaturatedly in ferrite isfixed to dislocation in the ferrite which has been introduced duringworking, with consequent increase in yield strength of the steel sheet,thus leading to improvement of BH property.

[0009] However, since there is a limit to the amount of the solidsolution C capable of being present supersaturatedly in ferrite, it isdifficult to attain a predetermined or higher BH property. For example,there is a problem such that a large deformation results in markeddeterioration of BH property, not affording a sufficient strength. Forexample, also in JP-A No. 297350/2000 there is disclosed a high tensilestrength hot-rolled steel sheet, but the present inventors have foundout that BH (10%) is about zero although BH (2%) is high.

DISCLOSURE OF THE INVENTION

[0010] The present invention has been accomplished in view of theabove-mentioned circumstances and it is a first object of the inventionto provide a high strength steel sheet superior in formability (stretchflange formability and total elongation) and a method which can producesuch a steel sheet efficiently. It is a second object of the presentinvention to provide a high strength steel sheet superior not only inthe aforesaid formabilitybut also in fatigue characteristic, i.e., ahigh strength steel sheet having well-balanced stretch flangeformability, total elongation and fatigue characteristic, and a methodwhich can produce such a steel sheet efficiently. It is a third objectof the present invention to provide a high strength steel sheet superiornot only in the aforesaid formability but also in bake hardeningproperty, and a method which can produce such a steel sheet efficiently.

[0011] A first high strength steel sheet according to the presentinvention which could achieve the above first object of the invention:

[0012] (1) contains the following chemical components in mass %:

[0013] C: 0.06 to 0.25%

[0014] Si+Al: 0.5 to 3%

[0015] Mn: 0.5 to 3%

[0016] P: 0.15% or less (not including 0%)

[0017] S: 0.02% or less (not including 0%), and

[0018] (2) has a structure comprising:

[0019] (2-1) a base phase structure, the base phase structure beingtempered martensite or tempered bainite and accounting for 50% or morein terms of a space factor relative to the whole structure, or the basephase structure comprising tempered martensite or tempered bainite whichaccounts for 15% or more in terms of a space factor relative to thewhole structure and further comprising ferrite, the tempered martensiteor the tempered bainite having a hardness which satisfies the relationof Vickers hardness (Hv)≧500[C]+30[Si]+3[Mn]+50 where [ ] represents thecontent (mass %) of each element, and

[0020] (2-2) a second phase structure comprising retained austenitewhich accounts for 3 to 30% in terms of a space factor relative to thewhole structure and optionally further comprising bainite and/ormartensite, the retained austenite having a C concentration (Cγ_(R)) of0.8% or more.

[0021] A second high strength steel sheet which could achieve theforegoing second object of the present invention:

[0022] (1) contains the following chemical components in mass %:

[0023] C: 0.06 to 0.25%

[0024] Si+Al: 0.5 to 3%

[0025] Mn: 0.5 to 3%

[0026] P: 0.15% or less (not including 0%)

[0027] S: 0.02% or less (not including 0%), and

[0028] (2) has a structure satisfying the structure of the first highstrength steel sheet described above, wherein the second phase structuresatisfies the following expression (1) to enhance a fatiguecharacteristic:

(S1/S)×100≧20   (1)

[0029] where S stands for a total area of the second phase structure,and S1 stands for a total area of coarse second phase crystal grains(Sb) contained in the second phase structure, the Sb corresponding tothree times or more as large as an average crystal grain area (Sm) ofthe second phase structure.

[0030] A third high strength steel sheet according to the presentinvention which could achieve the foregoing third object of the presentinvention:

[0031] (1) contains the following chemical components in mass %:

[0032] C: 0.06 to 0.25%

[0033] Si+Al: 0.5 to 3%

[0034] Mn: 0.5 to 3%

[0035] P: 0.15% or less (not including 0%)

[0036] S: 0.02% or less (not including 0%),

[0037] (2) has a structure satisfying the structure of the first highstrength steel sheet described above, and

[0038] (3) has a hardening property (BH) after baking finish whichproperty satisfies:

[0039] BH (2%)≧70 MPa and

[0040] BH (10%)≧BH (2%)/2.

[0041] A method for producing the first high strength steel sheetdescribed above involves the following methods according to thefollowing structures (A) and (B):

(A) Steel Sheet with a Base Phase Structure Being Tempered Martensite orTempered Bainite

[0042] In this case there may be adopted the following method (1) or(2):

[0043] (1) A method of producing the above steel sheet through a hotrolling process and a continuous annealing process or a plating process:

[0044] the hot rolling process comprising a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C. and a step ofcooling a resulting steel sheet to a temperature of not higher than Mspoint (in case of a base phase structure being tempered martensite) or atemperature of not lower than Ms point and not higher than Bs point (incase of a base phase structure being tempered bainite) at an averagecooling rate of not lower than 20° C./s and winding up the steel sheet,

[0045] the continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

[0046] (2) A method of producing the above steel sheet through a hotrolling process, a cooling process, a first continuous annealingprocess, and a second continuous annealing process or a plating process:

[0047] the first continuous annealing process comprising a step ofholding a resulting steel sheet in a heated state at a temperature ofnot lower than A₃ point and a step of cooling the steel sheet to atemperature of not higher than Ms point (in case of a base phasestructure being tempered martensite) or a temperature of not lower thanMs point and not higher than Bs point (in case of a base phase structurebeing tempered bainite) at an average cooling rate of not lower than 20°C./s, the second continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A1 point and not higher than A3 point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

(B) Steel Sheet with a Base Phase Structure Comprising TemperedMartensite and Ferrite or Comprising Tempered Bainite and Ferrite

[0048] In this case there may be adopted the following method (3) or(4):

[0049] (3) A method of producing the above steel sheet through a hotrolling process and a continuous annealing process or a plating process:

[0050] the hot rolling process comprising a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C., cooling aresulting steel sheet to a temperature of not higher than Ms point (inthe case of a base phase structure comprising tempered martensite andferrite) or a temperature of not lower than Ms point and not higher thanBs point (in the case of a base phase structure comprising temperedbainite and ferrite) at an average cooling rate of not lower than 10°C./s and winding up the steel sheet,

[0051] the continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

[0052] (4) A method of producing the above steel sheet through a hotrolling process, a cooling process, a first continuous annealingprocess, and a second continuous annealing process or a plating process:

[0053] the first continuous annealing process comprising a step ofholding a resulting steel sheet in a heated state at a temperature ofnot lower than A₁ point and not higher than A₃ point and a step ofcooling the steel sheet to a temperature of not higher thanMs point (inthe case of abase phase structure comprising tempered martensite andferrite) or a temperature of not lower than Ms point and not higher thanBs point (in the case of a base phase structure comprising temperedbainite and ferrite) at an average cooling rate of not lower than 10°C./s,

[0054] the second continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A, point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

[0055] A method for producing the foregoing second high strength steelsheet involves the following methods according to the followingstructures (A) and (B):

(A) Steel Sheet with a Base Phase Structure Being Tempered Martensite orTempered Bainite

[0056] In this case there may be adopted the following method (5) or(6):

[0057] (5) A method of producing the above steel sheet through a hotrolling process, a tempering process, and a continuous annealing processor a plating process,

[0058] the hot rolling process comprising a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C. and a step ofcooling a resulting steel sheet to a temperature of not higher than Ms(in case of a base phase structure being tempered martensite) or atemperature of not lower than Ms and not higher than Bs (in case of abase phase structure being tempered bainite) at an average cooling rateof not lower than 20° C./s and winding up the steel sheet,

[0059] the tempering process comprising a step of tempering the steelsheet at a temperature of not lower than 400° C. and not higher thanA_(c1) point for a period of time of not less than 10 minutes and lessthan 2 hours,

[0060] the continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

[0061] (6) A method of producing the above steep sheet through a hotrolling process, a cooling process, a first continuous annealingprocess, a tempering process, and a second continuous annealing processor a plating process,

[0062] the first continuous annealing process comprising a step ofholding a resulting steel sheet in a heated state at a temperature ofnot lower than A₃ point and a step of cooling the steel sheet to atemperature of not higher than Ms point (in case of a base phasestructure being tempered martensite) or a temperature of not lower thanMs point and not higher than Bs point (in case of a base phase structurebeing tempered bainite) at an average cooling rate of not lower than 20°C./s,

[0063] the tempering process comprising a step of tempering the steelsheet at a temperature of not lower than 400° C. and not higher thanA_(c1) point for a period of time of not less than 10 minutes and lessthan 2 hours,

[0064] the second continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

[0065] (B) Steel Sheet with a Base Phase Structure Comprising TemperedMartensite and Ferrite or Comprising Tempered Bainite and Ferrite

[0066] In this case there may be adopted the following method (7) or(8):

[0067] (7) A method of producing the above steel sheet through a hotrolling process, a tempering process, and a continuous annealing processor a plating process,

[0068] the hot rolling process comprising a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C. and a step ofcooling a resulting steel sheet to a temperature of not higher than Mspoint (in the case of a base phase structure comprising temperedmartensite and ferrite) or a temperature of not lower than Ms point andnot higher than Bs point (in the case of a base phase structurecomprising tempered bainite and ferrite) at an average cooling rate ofnot lower than 10° C./s and winding up the steel sheet,

[0069] the tempering process comprising a step of tempering the steelsheet at a temperature of not lower than 400° C. and not higher than A₁point for a period of time of not less than 10 minutes and less than 2hours,

[0070] the continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

[0071] (8) A method of producing the above steel sheet through a hotrolling process, a cooling process, a first continuous annealingprocess, a tempering process, and a second continuous annealing processor a plating process,

[0072] the first continuous annealing process comprising a step ofholding a resulting steel sheet in a heated state at a temperature ofnot lower than A₁ point and not higher than A₃ point and a step ofcooling the slab to a temperature of not higher than Ms point (in thecase of a base phase structure comprising tempered martensite andferrite) or a temperature of not lower than Ms point and not higher thanBs point (in the case of a base phase structure comprising temperedbainite and ferrite) at an average cooling rate of not lower than 10°C./s,

[0073] the tempering process comprising a step of tempering the steelsheet at a temperature of not lower than 400° C. and not higher thanA_(c1) point for a period of time of not less than 10 minutes and lessthan 2 hours,

[0074] the second continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C., and a step ofholding the steel sheet in the said temperature range for 1 second ormore.

[0075] A method for producing the foregoing third high strength steelinvolves the following methods according to the following structures (A)and (B):

(A) Steel Sheet with a Base Phase Structure Being Tempered Martensite orTempered Bainite

[0076] In this case there may be adopted the following method (9) or(10):

[0077] (9) A method of producing the above steel sheet through a hotrolling process and a continuous annealing process or a plating process,

[0078] the hot rolling process comprising a step of controlling aheating temperature before hot rolling to a temperature of 950° to 1000°C., a step of terminating finish rolling at a temperature of not lowerthan (A_(r3)−50)° C., and a step of cooling a resulting steel sheet to atemperature of not higher than Ms point or a temperature of not lowerthan Ms point and not higher than Bs point at an average cooling rate ofnot lower than 20° C./s and winding up the steel sheet,

[0079] the continuous annealing process comprising a step of holding thesteel sheet in a heated state at a temperature of not lower than A₁point and not higher than A₃ point for 10 to 600 seconds, a step ofcooling the steel sheet to a temperature of not lower than 300° C. andnot higher than 480° C. at an average cooling rate of not lower than 3°C./s, and a step of holding the steel sheet in the said temperaturerange for 1 second or more. (10) A method of producing the above steelsheet through a hot rolling process, a cooling process, a firstcontinuous annealing process, and a second continuous annealing processor a plating process,

[0080] the hot rolling process comprising a step of controlling aheating temperature before hot rolling to a temperature of 950° to 1100°C.,

[0081] the first continuous annealing process comprising a step ofholding a resulting steel sheet in a heated state at a temperature ofnot lower than A₃ point and a step of cooling the steel sheet to atemperature of not higher than Ms point or a temperature of not lowerthan Ms point and not higher than Bs point at an average cooling rate ofnot lower than 20° C./s,

[0082] the second continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet inthe said temperature range for 1 second or more.

(B) Steel Sheet with a Base Phase Structure Comprising TemperedMartensite and Ferrite or Comprising Tempered Bainite and Ferrite

[0083] In this case there may be adopted the following method (11) or(12):

[0084] (11) A method of producing the above steel sheet through a hotrolling process and a continuous annealing process or a plating process,

[0085] the hot rolling process comprising a step of controlling aheating temperature before hot rolling to a temperature of 950° to 1100°C., a step of terminating finish rolling at a temperature of not lowerthan (A_(r3)−50)° C., and a step of cooling a resulting steel sheet to atemperature of not higher than Ms point or a temperature of not lowerthan Ms point and not higher than Bs point at an average cooling rate ofnot lower than 10° C./s, the continuous annealing process or the platingprocess comprising a step of holding the steel sheet in a heated stateat a temperature of not lower than A₁ point and not higher than A₃ pointfor 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in the said temperature range for 1 second or more.

[0086] (12) A method of producing the above steel sheet through a hotrolling process, a cooling process, a first continuous annealingprocess, and a second continuous annealing process or a plating process,

[0087] the hot rolling process comprising a step of controlling aheating temperature before hot rolling to a temperature of 950° to 1100°C.,

[0088] the first continuous annealing process comprising a step ofholding a resulting steel sheet in a heated state at a temperature ofnot lower than A₁ point and not higher than A₃ point and a step ofcooling the steel sheet to a temperature of not higher than Ms point ora temperature of not lower than Ms point and not higher than Bs point atan average cooling rate of not lower than 10° C./s,

[0089] the second continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C., and a step ofholding the steel sheet in the said temperature range for 1 second ormore.

BRIEF DESCRIPTION OF THE DRAWINGS

[0090]FIG. 1 is a graph comparing between the hardness of temperedmartensite and that of polygonal ferrite in the same component system;

[0091]FIG. 2 is a graph showing the influence of the amount of C on thehardness of tempered martensite and that of polygonal ferrite;

[0092]FIG. 3 schematically illustrates characteristics of retainedaustenite (γ_(R)) in the present invention;

[0093]FIG. 4 is an EBSP photograph (×1000) of a steel sheet (No. 3 inTable 2) according to the present invention;

[0094]FIG. 5 is an EBSP photograph (×1000) of a conventional retainedaustenite steel sheet (No. 16 in Table 3);

[0095]FIG. 6 illustrates the hot rolling process in the method (1), (3),(5), (7), (9), or (11) in the case where a base phase structure istempered martensite or comprises tempered martensite and ferrite;

[0096]FIG. 7 illustrates the hot rolling process in the method (1), (3),(5), (7), (9), or (11) in the case where a base phase structure istempered bainite or comprises tempered bainite and ferrite;

[0097]FIG. 8 illustrates the continuous annealing process or the platingprocess in the method (1), (3), (5), (7), (9), or (11);

[0098]FIG. 9 illustrates the first continuous annealing process in themethod (2), (6), or (10) in the case where a base phase structure istempered martensite;

[0099]FIG. 10 illustrates the first continuous annealing process in themethod (2), (6), or (10) in the case where a base phase structure istempered bainite;

[0100]FIG. 11 illustrates the first continuous annealing process in themethod (4), (8), or (12) in the case where a base phase structurecomprises tempered martensite and ferrite;

[0101]FIG. 12 illustrates the first continuous annealing process in themethod (4), (8), or (12) in the case where a base phase structurecomprises tempered bainite and ferrite;

[0102]FIG. 13 is a TEM photograph of No. 3 in Example 1;

[0103]FIG. 14 is a TEM photograph of No. 3 in Example 2;

[0104]FIG. 15 is a TEM photograph of No. 3 in Example 3;

[0105]FIG. 16 is a TEM photograph of No. 3 in Example 4;

[0106]FIG. 17 is an optical microphotograph of No. 3 in Example 5;

[0107]FIG. 18 is an optical microphotograph of No. 3 in Example 6;

[0108]FIG. 19 is an optical microphotograph of No. 3 in Example 7;

[0109]FIG. 20 is an optical microphotograph of No. 3 in Example 8;

[0110]FIG. 21 is an SEM photograph (×4000) of No. 13 in Table 32; and

[0111]FIG. 22 is an SEM photograph (×4000) of No. 12 in Table 32.

BEST MODE FOR CARRYING OUT THE INVENTION

[0112] First, the following description is provided about the first highstrength steel sheet according to the present invention.

[0113] Having made earnest studies for providing a low alloy TRIP steelsheet having a high stretch flange formability and a high totalelongation, the present inventors found out that the desired objectcould be achieved by using as a base phase structure {circle over (1)}tempered martensite or {circle over (2)} tempered bainite, which is asoft lath structure low in dislocation density, or {circle over (3)} amixed structure of the tempered martensite and ferrite, or {circle over(4)} a mixed structure of the tempered bainite and ferrite, and bymaking control to, as a second phase structure, a structure having γ_(R)phase with a C concentration (Cγ_(R)) in retained austenite (γ_(R)) ofnot lower than 0.8%. On the basis of this finding we have accomplishedthe present invention.

[0114] A description will be given below about the base phase structureand the second phase structure which are the greatest feature of thefirst high strength steel sheet. These structures are not only presentin the first steel sheet but also present in common to the second andthe third steel sheet which will be described later.

(1) Base Phase Structure {circle over (1)} A Mode Using a TemperedMartensite Structure as a Base Phase Structure

[0115] A conventional retained austenite steel sheet has a demerit suchthat with progress of deformation of a soft phase (base phase) around ahard phase, voids are apt to occur in the interface with the soft phase,resulting in stretch flange formability being deteriorated. But by usingnot ferrite in the prior art but tempered martensite (or temperedbainite, or a mixed structure of tempered martensite and ferrite, or amixed structure of tempered bainite and ferrite, which will be describedlater) as a base phase, the formation of voids has been suppressed andstretch flange formability improved. Further, by controlling the form oflath γ_(R) so as to give a predetermined axial ratio, it has becomepossible to improve elongation and stretch flange formability ascompared with the conventional γ_(R).

[0116] “Tempered martensite” used in the present invention has thefollowing features.

[0117] Firstly, “tempered martensite” in the present invention means asoft and lath structure low in dislocation density. On the other hand,martensite is a hard structure high in dislocation density and isdifferent in this point from the tempered martensite. Both can bedistinguished from each other, for example, by observation under atransmission electron microscope (TEM). A conventional γ_(R) steel sheethas a soft block-like ferrite structure low in dislocation density andis also different in this point from the steel sheet of the presentinvention which uses the tempered martensite as a base phase structure.

[0118] Secondly, the tempered martensite has a tendency that its Vickershardness (Hv) is generally high as compared with polygonal ferrite inthe same component system (a system common in point of basic componentsof C, Si, and Mn). FIG. 1 is a graph comparing between the hardness oftempered martensite (axis of ordinate) and that-of polygonal ferrite(axis of abscissa) in steels of the same components (C: 0.1 to 0.3%, Mn:1.0 to 2.0%, Si: 1.0 to 2.0%). As to Vickers hardness, there was madeobservation through an optical microscope for Lepera etching andVickershardness (Hv) of abasephase (gray) portion was measured (load: 1 g). Forreference, a straight line y=x is shown with a dotted line in the samefigure, from which it is seen that the hardness of tempered martensiteis higher than that of polygonal ferrite and that such a tendencybecomes more outstanding as the hardness becomes higher.

[0119] In FIG. 2, the data of FIG. 1 are arranged for each of the casesof C being 0.1%, 0.2%, and 0.3%, showing the influence of the amount ofC on the hardness of tempered martensite and that of polygonal ferrite.From FIG. 2 it is seen that, in the same amount of C, the hardness oftempered martensite tends to be higher than that of polygonal ferriteand that this tendency becomes outstanding as the amount of C becomeshigher.

[0120] If the hardness of tempered martensite and that of polygonalferrite are expressed in terms of relations to the basic components ofC, Mn, and Si on the basis of the above results, the following relationsare obtained:

[0121] Hardness (Hv) of tempered martensite

≧500[C]+30[Si]+3[Mn]+50

[0122] Hardness (Hv) of polygonal ferrite

≈200[C]+30[Si]+3[Mn]+50

[0123] where, [ ] represents the content (mass %) of each element.

[0124] We have confirmed that the hardness values (calculated values)obtained from the above relations reflect measured values.

[0125] We have also confirmed that the hardness values obtained from theabove relations reflect measured values not only in case of the amountof C being 0.1 to 0.3% but also in case of the amount of C being 0.3 to0.6%, further, 0.06 to 0.1%.

[0126] An upper limit in hardness of tempered martensite can varydepending on a component composition for example, but it is recommendedthat the said upper limit be approximately 500[C]+30[Si]+3[Mn]+200,preferably 500[C]+30[Si]+3[Mn]+150.

[0127] As will be described later, tempered martensite having such acharacteristic is obtained by providing martensite which has beenquenched from a temperature of not lower than A₃ point (γ region) andannealing the martensite at a temperature of not lower than A₁ point(about 700° C. or higher) and not higher than A₃ point.

[0128] For allowing the effect of improving the stretch flangeformability by the tempered martensite to be exhibited effectively, itis necessary that the tempered martensite be present not less than 50%(preferably not less than 60%) in terms of a space factor relative tothe whole structure. The amount of the tempered martensite is determinedin consideration of its balance with γ_(R). It is recommended forcontrol to be made appropriately so that a desired characteristic can beexhibited.

{circle over (2)} A Mode Using a Mixed Structure of Tempered Martensiteand Ferrite as a Base Phase Structure

[0129] In this mode, the details of tempered martensite is as describedabove in {circle over (1)}. In this mixed base phase structure, in orderfor the tempered martensite to function effectively, it is necessarythat the tempered martensite be present not less than 15% (preferablynotless than 20%) interms of a space factor relative to the wholestructure. The amount of the tempered martensite is determined, takinginto account the balance of ferrite and γ_(R) which will be describedlater. It is recommended for control to be made appropriately so that adesired characteristic can be exhibited.

[0130] The term “ferrite” as referred to hereinmeans polygonal ferrite,i.e., ferrite low in dislocation density. The ferrite is superior inelongation characteristic but is inferior in stretch flange formability.On the other hand, a steel sheet according to the present inventionhaving the foregoing mixed structure of ferrite and tempered martensiteis improved in stretch flange formability while retaining an excellentelongation characteristic. Thus, in both structural construction andresulting characteristics the steel sheet of the present invention isdifferent from the conventional TRIP steel sheet.

[0131] In order for the action based on the present invention to beexhibited effectively it is recommended that ferrite be present not lessthan 5% (preferably not less than 10%) in terms of a space factorrelative to the whole structure. However, if the content of ferriteexceeds 60%, it will become difficult to ensure a required strength;besides, like the conventional TRIP steel sheet, therewilloccurmanyvoids from the interface between ferrite and a second phase,with consequent deterioration of the stretch flange formability. It istherefore recommended that the upper limit of ferrite content be set at60%. Controlling the upper limit to less than 30% is very preferablebecause the ferrite-second phase (γ_(R), martensite) interface willdiminish to suppress the formation of voids, thus leading to improvementof the stretch flange formability.

{circle over (3)} A Mode Using Tempered Bainite as a Base PhaseStructure

[0132] “Tempered bainite” used in the present invention has thefollowing features.

[0133] Firstly, “tempered bainite” in the present invention means a softand lath structure low in dislocation density. On the other hand,bainite is a hard structure high in dislocation density and is differentin this point from the tempered bainite. Both can be distinguished fromeach other, for example, by observation under a transmission electronmicroscope (TEM). A conventional γ_(R) steel sheet has a soft block-likesoft structure low in dislocation density and is also different in thispoint from the steel sheet of the present invention which uses thetempered bainite as a base phase structure.

[0134] Secondly, the tempered bainite has a tendency that its Vickershardness (Hv) is generally high as compared with polygonal ferrite inthe same component system (a system common in point of basic componentsof C, Si, and Mn). FIG. 1 is a graph comparing the hardness of temperedbainite and that of tempered martensite (axis of ordinate) with thehardness of polygonal ferrite (axis of abscissa) in steels of the samecomponents (C: 0.1 to 0.3%, Mn: 1.0 to 2.0%, Si: 1.0 to 2.0%). As toVickers hardness, there was made observation through an opticalmicroscope for Lepera etching and Vickers hardness of a base phase(gray) portion was measured (load: 1 g). For reference, a straight liney=x is shown with a dotted line in the same figure, from which it isseen that the hardness of tempered martensite is higher than that ofpolygonal ferrite and that such a tendency becomes more outstanding asthe hardness becomes higher.

[0135] In FIG. 2, the data of FIG. 1 are arranged for each of the casesof C being 0.1%, 0.2%, and 0.3%, showing the influence of the amount ofC on the hardness of tempered bainite, tempered martensite, andpolygonal ferrite. From FIG. 2 it is seen that, in the same amount of C,the hardness of tempered bainite tends to be higher than that ofpolygonal ferrite and that this tendency becomes more outstanding as theamount of C increases.

[0136] On the basis of these results, if the hardness of temperedbainite and that of polygonal ferrite are expressed in terms ofrelations to the basic components of C, Mn, and Si, there are obtainedthe following relations:

[0137] Hardness (Hv) of tempered bainite

≧500[C]+30[Si]+3[Mn]+50

Hardness (Hv) of polygonal ferrite

≈299[C]+30[Si]+3[Mn]+50

[0138] where, [ ] represents the content (mass %) of each element.

[0139] We have confirmed that the hardness values (calculated values)obtained from the above relations reflect measured values.

[0140] We have also confirmed that the hardness values obtained from theabove relations reflect measured values not only in case of the amountof C being 0.1 to 0.3% but also in case of the amount of C being 0.3 to0.6%, further, 0.06 to 0.1%.

[0141] An upper limit in hardness of tempered bainite can vary dependingon a component composition for example, but it is recommended that thesaid upper limit be approximately 500[C]+30[Si]+3[Mn]+200, preferably500[C]+30[Si]+3[Mn]+150.

[0142] As will be described later, tempered bainite having such acharacteristic is obtained by providing bainite which has been quenchedfrom a temperature of not lower than A3 point (γ region) to atemperature of not lower than Ms point and not higher than Bs point andby annealing the bainite at a temperature of not lower than A₁ point(about 700° C. or higher) and not higher than A₃ point.

[0143] For allowing the effect of improving the stretch flangeformabilitybythetemperedbainitetobeexhibitedeffectively, it isrecommended that the tempered bainite be present not less than 50%(preferablynot less than 60%) in terms of a space factor relative to thewhole structure. The amount of the tempered bainite is determined inconsideration of its balance with γ_(R) which will be described later.It is recommended for control to be made appropriately so that a desiredcharacteristic can be exhibited.

{circle over (4)} A Mode Using a Mixed Structure of Tempered Bainite andFerrite as a Base Phase Structure

[0144] The details of the structures (tempered bainite and ferrite) inthis mode are as described in the above {circle over (3)} and {circleover (2)}.

[0145] In this mixed based phase structure, in order for the temperedbainite to function effectively, it is necessary that the temperedbainite be present not less than 15% (preferably not less than 20%) interms of a space factor relative to the whole structure. The amount ofthe tempered bainite is determined, taking into account the balance offerrite and γ_(R) which will be described later. It is recommended forcontrol to be made appropriately so that a desired characteristic can beexhibited.

(2) Second Phase Structure

[0146] A description will be given below of the second phase structurein each of the above modes {circle over (1)} to {circle over (4)}.

Retained Austenite (γ_(R))

[0147] γ_(R) is effective in improving the fatigue characteristic and inorder for this function to be exhibited effectively it is necessary thatγ_(R) be present 3% (preferably 5% or more) in terms of a space factorrelative to the whole structure. Particularly, in the case where a basephase structure is a mixed structure of tempered martensite and ferrite,it is preferable that γ_(R) be present 5% or more (more preferably 7% ormore). If γ_(R) is present in a large amount, the stretch flangeformability will be deteriorated. Therefore, we have determined an upperlimit of the γ_(R) content to be 30%. Especially when a base phasestructure is a single phase structure of tempered martensite or temperedbainite, it is recommended that the upper limit be controlled to 20%(more preferably 15%). On the other hand, if a base phase structure is amixed structure of tempered martensite and ferrite or a mixed structureof tempered bainite and ferrite, it is recommended to set the upperlimit at 25%.

[0148] Further, it is necessary that the concentration of C (Cγ_(R)) inthe γ_(R) be not less than 0.8%. The Cγ_(R) exerts a great influence onthe characteristic of TRIP (transformation induced plasticity), andcontrolling the Cγ_(R) to 0.8% or more will be effective particularly inimproving elongation, etc. Preferably, the Cγ_(R) is not less than 1%,more preferably not less than 1.2%. Although the higher the Cγ_(R), themore preferable, an adjustable upper limit in practical operation isconsidered to be approximately 1.6% In a conventional TRIP steel sheet,γ_(R) of random orientation is present in a pre-austenite grainboundary, while, in the present invention, γ_(R) having the sameorientation along for example a block boundary within the same packet isapt to be present. A feature of the γ_(R) in the present invention isillustrated schematically in FIG. 3. In the same figure, the numeral 1denotes a pre-austenite grain boundary, numeral 2 denotes a packet grainboundary, numeral 3 denotes a block grain boundary, and numeral 4denotes martensite lath.

[0149] For the purpose of making this point clearer, FIGS. 4 and 5illustrate results obtained using EBSP photographs (color maps:magnification 1000 times) of sections in sheet thickness direction of asteel sheet according to the present invention (No. 3 in Table 2 to bedescribed later) and a conventional γ_(R) steel sheet (No. 16 in Table 3to be described later). The EBSP stands for Electron Back ScatterDiffraction Pattern, and as an EBSP analyzer there was used an analyzermanufactured by TexSEM Laboratories.

[0150] With the photographs, γ_(R) in the sheet thickness direction ofdifferent crystal orientations can be identified on the basis of a colortone difference. That is, if γ_(R) is checkedbya crystal orientationobserving method using EBSP different from the ordinary structureobservation, a large number of γ_(R) of random orientations are found tobe present in a pre-austenite grain boundary in the conventional steelsheet (FIG. 5), while in the steel sheet according to the presentinvention (FIG. 4) it can be seen that a large number of γ_(R) havingthe same orientation are present within a certain region, though both ofthe steel sheet have almost the same structure in appearance. It ispresumed that, in the steel sheet of the present invention, γ_(R) havingthe same orientation is produced along a block boundary for example. Inthis point the γ_(R) in the steel of the present invention has adifferent form from the that in the conventional steel sheet.

[0151] It is preferable that the γ_(R) in the present invention be inlath form. By “lath form” is meant an average axial ratio (majoraxis/minor axis) of 2 or more (preferably 4 or more, a preferred upperlimit being 30 or less). The γ_(R) in lath form not only affords thesame TRIP effect as in the prior art but also affords improvedelongation and a more outstanding improvement in stretch flangeformability.

Others: Bainite and/or Martensite (including 0%)

[0152] In addition to the above retained austenite, the second phasestructure may further contain bainite and/or martensite as otherstructures insofar as the operation of the present invention is notimpaired. These structures may remain inevitably in the manufacturingprocess of the present invention, but the smaller their content, thebetter. In the second high strength steel sheet according to the presentinvention, which will be described later, mention may be made mainly ofmartensite as another structure.

[0153] Next, reference will be made to basic components which constitutethe steel sheet of the present invention. In the following description,the amounts of chemical components are all in mass %.

C: 0.06 to 0.6%

[0154] C is an element essential for ensuring a high strength and forensuring γ_(R) . More specifically, C is an important element forproviding a sufficient content of C in γ phase and for allowing adesiredyphase to remain even at roomtemperature. C is useful inimproving the balance of strength and stretch flange formability.Particularly, if C is added in an amount of 0.25% or more, the amount ofγ_(R) increases and C concentration to γ_(R) becomes higher, so thatthere can be obtained an extremely high strength-elongation balance.

[0155] However, if the amount of C added exceeds 0.6%, not only theeffect thereof will become saturated, but also there will occur a defectcaused by, for example, center segregation into casting. Moreover, if Cis added in an amount of 0.25% or more, a deterioration of weldabilitywill result.

[0156] Thus, if weldability is mainly taken into account, it ispreferable to control the amount of C to 0.06 to 0.25% (more preferably0.2% or less, still more preferably 0.15% or less) On the other hand, inthe case where high elongation is required without the need of spotwelding, it is recommended to control the amount of C to 0.25 to 0.6%(more preferably 0.3% or more)

Si+Al: 0.5 to 3%

[0157] Si and Al are elements which effectively prevent the formation ofcarbide by decomposition of γ_(R) . Especially, Si is useful also as asolid solution hardening element. For allowing such a function to beexhibited effectively it is necessary that Si and Al be added a total of0.5% or more, preferably 0.7% or more, more preferably 1% or more. Buteven if both elements are added in an amount exceeding 3% in total, theaforesaid effect will become saturated, which is wasteful from theeconomic standpoint; besides, the addition thereof in a large amountwill cause hot shortness. For this reason, an upper limit thereof is setat 3%, preferably 2.5% or less, more preferably 2% or less.

Mn: 0.5 to 3%

[0158] Mn is an element necessary for stabilizing γ and for obtaining adesired γ_(R). For allowing such a function to be exhibited effectivelyit is necessary to add Mn in an amount of 0.5% or more, preferably 0.7%or more, more preferably 1% or more. However, if Mn is added in anamount exceeding 3%,. there will arise a bad influence such as castpiece cracking. Preferably, Mn is added in an amount of not larger than2.5%, more preferably not larger than 2%.

P: 0.15% or Less (Not Including 0%)

[0159] P is an element effective for ensuring a desired γ_(R). Forallowing such a function to be exhibited effectively it is recommendedto add P in an amount of 0.03% or more (more preferably 0.05% or more).However, if the amount of P added exceeds 0.1%, secondary formabilitywill be deteriorated. More preferably, P is added in an amount of notlarger than 0.1%.

S: 0.02% or Less (Including 0%)

[0160] S is an element which forms a sulfide inclusion such as MnS andacts as an origin of cracking, with consequent deterioration offormability. The content of S is preferably not more than 0.02%, morepreferably not more than 0.015%.

[0161] The steel of the present invention basically contains the abovecomponents, with the balance being substantially iron and impurities,but the following components may be added insofar as they do not impairthe operation of the present invention: At least one of Mo: 1% or less(not including 0%), Ni: 0.5% or less (not including 0%), Cu: 0.5% orless (not including 0%), Cr: 1% or less (not including 0%)

[0162] These elements are not only useful as steel strengtheningelements but also effective in stabilizing γ_(R) and ensuring apredetermined amount thereof. For allowing such functions to beexhibited effectively, it is recommended that these elements be added insuch amounts as Mo: 0.05% or more (more preferably 0.1% or more), Ni:0.05% or more (more preferably 0.1% or more), Cu: 0.05% or more (morepreferably 0.1% or more), and Cr: 0.05% or more (more preferably 0.1% ormore). However, even if Mo and Cr are added in an amount exceeding 1%and Ni and Cu are added in an amount exceeding 0.5%, the above effectswill become saturated, which is wasteful from the economic standpoint.More preferably, these elements are added in such amounts as Mo: 0.8% orless, Ni: 0.4% or less, Cu: 0.4% or less, and Cr: 0.8% or less. At leastone of Ti: 0.1% or less (not including 0%), Nb: 0.1% or less (notincluding 0%), V: 0.1% or less (not including 0%)

[0163] These elements have a precipitation strengthening andmicrostructurization effect and are useful for the attainment of a highstrength. For allowing these functions to be exhibited effectively it isrecommended that these elements be added in such amounts as Ti: 0.01% ormore (preferably 0.02% or more), Nb: 0.01% or more (more preferably0.02% or more), and V: 0.01% or more (more preferably 0.02% or more).However, with respect to all of these elements, an amount exceeding 0.1%will result in saturation of the above effects, which is wasteful fromthe economic standpoint. More preferably, these elements are added insuch amounts as Ti: 0.08% or less, Nb: 0.08% or less, and V: 0.08% orless.

Ca: 0.003% or Less and/or REM: 0.003% or Less (Not Including 0%)

[0164] Ca and REM (rare earth elements) function to control the form ofsulfide in steel and are effective in improving formability. As examplesof rare earth elements employable in the present invention are mentionedSc, Y, and lanthanoid. For allowing the above effect to be exhibitedeffectively it is recommended that these elements be each added in anamount of 0.0003% or more (more preferably 0.0005% or more). However,even an amount thereof exceeding 0.003% would result in saturation ofthe above effect, which is wasteful from the economic standpoint. It ismore preferable that they each be added in an amount of 0.0025% or less.

[0165] Next, how to produce the foregoing first steel sheet will bedescribed below structure by structure.

(A) Steel Sheet with a Base Phase Structure Being Tempered Martensite orTempered Bainite

[0166] The following methods (1) and (2) are mentioned as typicalmethods for producing this steel sheet.

(1) [Hot Rolling Pgrocess]→[Continuous Annealing Process PlatingProcess]

[0167] This method produces a desired steel sheet through {circle over(1)} a hot rolling process or {circle over (2)} a continuous annealingprocess or a plating process. The hot rolling process {circle over (1)}is illustrated in FIG. 6 (in case of a base phase structure beingtempered martensite) and FIG. 7 (in case of a base phase structure beingquenched bainite), and the continuous annealing process or platingprocess {circle over (2)} is illustrated in FIG. 8.

{circle over (1)} Hot Rolling Process

[0168] The hot rolling process comprises a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C. and a step ofcooling the rolled steel sheet to a temperature of not higher than Mspoint (in case of abase phase structure being tempered martensite) or atemperature of not lower than Ms point and not higher than Bs point (incase of a base phase structure being tempered bainite) at an averagecooling rate of not lower than 20° C./s and winding up the steel sheet.The hot rolling conditions have been established for obtaining a desiredbase phase structure (quenched martensite or quenched bainite).

[0169] No matter which base phase structure may be adopted, it isrecommended that a hot rolling finish temperature (FDT) be set at atemperature of not lower than (A_(r3)−50)° C., preferably not lower thanA_(r3) point. This is for obtaining a desired quenched martensite orquenched bainite in cooperation with the “cooling to not higher than Mspoint” or “cooling to not lower than Ms point and not higher than Bspoint” which follows the hot rolling process.

[0170] It is recommended that the cooling, which follows the hot rollingprocess, be carried out to a temperature of not higher than Ms point atan average cooling rate of not lower than 20° C./s while avoidingferrite transformation and pearlite transformation. This enables adesired quenched martensite or quenched bainite to be obtained withoutformation of polygonal ferrite, etc. The average cooling rate after thehot rolling also exerts an influence on the final form of γ_(R). If theaverage cooling rate is high, there will be obtained a lath form. Anupper limit of the average cooling rate is not specially limited, andthe higher, the better. But in relation to the actual operation level itis recommended to make control appropriately.

[0171] In the case where quenched martensite is to be obtained, it isnecessary that the winding temperature (CT) be set at a temperature ofnot higher than Ms point [calculating expression:Ms=561−474×[C]−33×[Mn]−17×[Ni]−17×[Cr]−21×[Mo], where [ ] representsmass % of each element. This is because, if the winding temperatureexceeds Ms point, it is impossible to obtain a desired quenchedmartensite and there are produced bainite, etc.

[0172] On the other hand, when quenched bainite is to be obtained, it isnecessary that the winding temperature (CT) should be not lower than Mspoint and not higher than Bs point [calculating expression: theexpression of Ms is the same as above;Bs=830−270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−80×[Mo], where represents mass %of each element]. This is because, if the winding temperature exceeds Bspoint, a desired quenched bainite is not obtained, while if it is lowerthan Ms point, there is produced tempered martensite.

[0173] In the hot rolling process it is recommended that each of theforegoing steps be controlled appropriately in order to obtain a desiredquenched martensite or quenched bainite. But as to other conditions,including the heating temperature, there maybe selected conventionalconditions (e.g., about 1000 to 1300° C.) suitably.

{circle over (2)} Continuous Annealing Process or Plating Process

[0174] The above hot rolling process {circle over (1)} is followed bycontinuous annealing or plating. However, if the shape after the hotrolling is not satisfactory, then for the purpose of correcting theshape there may be applied a cooling process after the hot rolling{circle over (1)} and before the continuous annealing or plating {circleover (1)}. In this case, it is recommended that the cold rolling rate beset at 1 to 30%. This is because, if cold rolling is carried out at acold rolling rate exceeding 30%, the rolling load will increase and itwill become difficult to effect cold rolling.

[0175] The continuous annealing or plating process comprises a step ofholding the steel sheet in a heated state at a temperature of not lowerthan A₁ point and not higher than A₃ point for 10 to 600 seconds, a stepof cooling the steel sheet to a temperature of not lower than 300° C.and not higher than 480° C. at an average cooling rate of not lower than3° C./s, and a step of holding the steel sheet in the said temperaturerange for 1 second or more. These conditions have been established fortempering the base phase structure (quenched martensite or quenchedbainite) produced in the hot rolling process to afford not only adesired tempered martensite but also a fine second phase.

[0176] First, by soaking at a temperature of not lower than A₁ point andnot higher than A₃ point (T3 in FIG. 8) for 10 to 600 seconds (t3 inFIG. 8) there is produced a desired structure (tempered martensite andγ_(R), or tempered bainite and γ_(R)) (annealing in two phase region).This is because, if the soaking temperature exceeds the abovetemperature range, the resulting product will all be γ, while if it islower than the above temperature range, it will be impossible to obtainthe desired γ_(R). Further, controlling the above heating holding time(t3) is particularly important for obtaining the desired structure. Thisis because, if the holding time is shorter than 10 seconds, temperingwill be insufficient and there will not be obtained the desired basephase structure (tempered martensite or tempered bainite). Preferably,the holding time is not shorter than 20 seconds, more preferably notshorter than 30 seconds. If the holding time exceeds 600 seconds, itwill become impossible to maintain the lath structure which is a featureof tempered martensite or tempered bainite, with consequentdeterioration of mechanical characteristics. Preferably, the holdingtime is not longer than 500 seconds, more preferably not longer than 400seconds.

[0177] Next, cooling is made to a temperature (bainite transformation:T4 in FIG. 8) of not lower than 300° C. (preferably not lower than 350°C.) and not higher than 480° C. (preferably not higher than 450° C.)while controlling an average cooling rate (CR) to a temperature of notlower than 3° C./s (preferably not lower than 5° C./s) and whileavoiding pearlite transformation, and the steel sheet is held in thistemperature range for 1 second or more (preferably 5 seconds or more: t4in FIG. 8), whereby the concentration of C to γ_(R) can be attained in alarge quantity and in an extremely short time.

[0178] If the average cooling speed is lower than the above range, thedesired structure will not be obtained, with formation of pearlite. Anupper limit of the average cooling rate is not specially limited and thehigher, the better. However, in relation to the actual operation levelit is recommended that control be made appropriately.

[0179] For allowing a desired amount of Cγ to be produced efficientlyduring cooling it is recommended to adopt a two-step cooling methodcomprising {circle over (1)} a step of cooling the steel sheet to atemperature (Tq) of (A₁ point to 600° C.) at an average cooling rate ofnot higher than 15° C./s and {circle over (2)} a step of cooling thesteel sheet to a temperature of not lower than 300° C. and not higherthan 480° C. at an average cooling rate of not lower than 20° C./s.

[0180] If cooling is performed to the above temperature range {circleover (1)} at an average cooling rate of not higher than 15° C./s(preferably not higher than 10° C./s), C is concentrated to γ in alarger amount. Next, if cooling is performed to the above temperaturerange {circle over (2)} at an average cooling rate of not lower than 20°C./s (preferably not lower than 30° C./s, more preferably not lower than40° C./s), the transformation of γ into pearlite is suppressed and γremains behind even at a low temperature. As a result, there is obtaineda desired γ structure. An upper limit of the average cooling rate is notspecially limited. The higher, the more desirable. However, in relationto the actual operation level, it is recommended to control the upperlimit appropriately.

[0181] The above cooling is followed by austempering. The austemperingtemperature (T4) is important for ensuring a desired structure andallowing the present invention to fulfill its operation. If theaustempering temperature is controlled to a temperature in the foregoingrange, there will be obtained γ_(R) stably in a large quantity, wherebyTRIP effect based on γ_(R) is exhibited. In contrast therewith, if theaustempering temperature is lower than 300° C., martensite phase willexist, while if it exceeds 480° C., the amount of bainite phase willincrease to a great extent.

[0182] An upper limit of the holding time (t4) is not specially limited,but if the time taken for transformation of austenite into bainite istaken into account, it is recommended to control the upper limit to atime of not longer than 3000 seconds, preferably not longer than 2000seconds.

[0183] In the above process, bainite structure may be produced insofaras it does not impair the operation of the present invention, inaddition to the desired base phase structure (tempered martensite ortempered bainite) and martensite. Further, plating and alloying may beperformed insofar as the desired structure is not decomposed markedlynor does the application of plating and alloying impair the operation ofthe present invention.

[0184] For producing an alloyed, hot dip galvanized steel sheet it isrecommended to carry out a predetermined Fe pre-plating prior to theabove plating. This for the following reason. This causes an Fe platedlayer not affected by surface concentration of Si to be formed on thesteel sheet surface and the number of coarse Zn—Fe alloy crystal grainspresent on the alloyed, hot dip galvanized layer surface is decreased toa remarkable extent. Thus, even at a low temperature, alloying iscarried out quickly by diffusion of the steel sheet and the Zn platedlayer, whereby not only γ_(R). which is effective in obtaining a highelongation characteristic stable, is obtained efficiently, but also itis possible to prevent the occurrence of disadvantages caused by theaddition of a large amount of Si [e.g., deterioration of powderingresistance caused by Si oxide, failure to effect plating, deteriorationin sliding property (slip characteristic) of the plated surface].

[0185] The coarse Zn—Fe alloy crystal grains present on the alloyed, hotdip galvanized layer surface mean Zn—Fe alloy crystal grains each havinga major side twice as long as a minor side, or less, and having anaverage grain diameter of 4 μm or more. By Fe pre-plating it is possibleto decrease the number of such coarse crystal grains to five or less(preferably three or less)/70 μm×50 μm. The average grain diameter ofthe Zn—Fe alloy crystal grains is determined by observing the alloyedlayer surface through an SEM (scanning electron microscope) (1500×) andcalculating an average length between a length measured in a largestlength direction of the crystal grains present in a visual field of 70μm×50 μm and a length in a direction orthogonal thereto.

[0186] More specifically, the above (a) Fe pre-plating is carried outbefore the steel sheet passes a continuous plating line [a series ofsuch line as CGL: annealing→(b) hot dip galvanizing (same as the above{circle over (1)})→alloying].

[0187] The steps (a) to (c) will be described below.

(a) Fe Pre-Plating

[0188] The pre-plating step (a) is carried out under conditions whichsatisfy the following relation (1):

0.06W≦X   (1)

[0189] where W stands for the amount of hot dip Zn plating deposited(g/m²) and X stands for the amount of Fe pre-plating deposited (g/m²)

[0190] First, the amount (X) of Fe pre-plating is controlled to a valueof not smaller than 0.06 W in relation to the amount (W) of hot dip Znplating deposited. This is because, if X is less than 0.06 W, Siconcentrates on the steel sheet surface as alloying proceeds, causingthe formation of coarse Zn—Fe alloy crystal grains which exert a badinfluence on the sliding property of the plated surface. Preferably, Xis 0.08 W or more, more preferably 0.10 W or more. An upper limit of Wis not specially limited from the standpoint of improving the slidingproperty of the plated surface, but if X is too much, an increase ofcost and deterioration of productivity will result. Therefore, it isrecommended to control the upper limit to 0.30 W, preferably 0.28 W orless, more preferably 0.25 W or less.

[0191] For effecting Fe pre-plating under conditions which satisfy theforegoing relation (1), it is recommended to carry out the conventionalplating while paying attention to electrolysis time. To be morespecific, it is recommended to set a plating bath composition toFeSO₄.7H₂O: 300 to 450 g/L), a plating bath pH to 1.7 to 2.6, a platingliquid temperature to 40 to 70° C., a current density to 10 to 250A/dm², and control the electrolysis time appropriately in accordancewith a desired amount of plating to be deposited.

[0192] Since the Fe pre-plating is followed by hot dip galvanizing andsubsequent alloying, the Fe pre-plating vanishes in the plated surfacelayer portion, but at the interface between the steel sheet and thealloyed, hot dip galvanized layer there may remain the Fe pre-platinglayer insofar as it does not impair the operation of the presentinvention.

(b) Hot Dip Galvanizing

[0193] The Fe plating is followed by annealing and subsequent hot dipgalvanizing referred to in the above {circle over (2)}. The detailedthereof are as described in the above {circle over (2)}.

[0194] In the hot dip galvanizing step it is recommended that aneffective Al concentration in the plating bath be controlled to a valuein the range of 0.08 to 0.12 mass % and the plating bath temperature toa temperature in the range of 4450 to 500° C. This is because alloyingis accelerated and powdering resistance is improved remarkably thereby.

[0195] First, it is preferable that an effective Al concentration in theplating bath be controlled to 0.08 to 0.12%. The “effective Alconcentration in the plating bath” means the concentration free Alcontained in the plating bath and in more detail it is represented bythe following expression:

[Effective Al concentration]=[Total Al concentration]−[Fe concentration(%) in the plating bath]

[0196] Generally, in the hot dip galvanizing step the effective Alconcentration in the plating bath is controlled to a value in the rangeof about 0.08 to 0.14%. However, in the above series of methods (a) to(c) the alloying temperature is set low for the purpose of obtaining adesired γ_(R), which will be described later. Therefore, alloying nolonger takes place as the Al concentration becomes higher. In thepresent invention, therefore, the upper limit of Al concentration iscontrolled preferably to 0.12% (more preferably 0.11%). However, if theAl concentration is lower than 0.08%, a lowering of powdering resistancewill result. More preferably, the Al concentration is not lower than0.09%.

[0197] It is preferable that the plating bath temperature be controlledto a temperature in the range of 445° to 500° C. A general plating bathtemperature is 430° to 500° C., but in the present invention, since Siwhich suppresses alloying is added in a large amount, the plating bathtemperature range is set to the above range for the purpose ofaccelerating alloying and enhancing the powdering resistance. If theplating bath temperature is lower than 445° C., there will remain an ηlayer (pure zinc). More preferably, the plating bath temperature is notlower than 450° C. On the other hand, a plating bath temperatureexceeding 500° C. will result in a lowering of powdering resistance.More preferably, the plating bath temperature is not higher than 490° C.

(c) Alloying

[0198] It is recommended that alloying be carried out at a temperatureof 400° to 470° C. for 5 to 100 seconds. If the alloying temperature islower, the alloying will slow down, with consequent deterioration ofproductivity. On the other hand, if the alloying temperature is higher,γ_(R) once produced will vanish. If the alloying time is shorter,alloying does not take place and there will remain an η layer (purezinc) on the surface. Conversely, a longer alloying time will lead to alowering of productivity.

[0199] Although reference has been made above to preferred modes whichgo through Fe pre-plating in the production of an alloyed, hot dipgalvanized steel sheet, the Fe pre-plating is applicable not only to theproduction of an alloyed hot dip galvanized steel sheet but also to theproduction of a ht dip galvanized steel sheet. More specifically, inproducing a hot dip galvanized steel sheet, if the foregoing (a) Fepre-plating and (b) hot dip galvanizing are performed, an Fe platedlayer not affected by surface concentration of Si is formed on the steelsheet surface, so that not only there is efficiently obtained γ_(R)which is effective in obtaining a high elongation characteristic, butalso the occurrence of disadvantages caused by the addition of a largeamount of Si can be prevented. Thus, the application of the platingsteps in question is extremely useful.

(2) [Hot Rolling Process]→[Cold Rolling Process]→[First ContinuousAnnealing Process]→[Second Continuous Annealing Process or PlatingProcess]

[0200] This method produces a desired steel sheet through a hot rollingprocess, a cooling process, a first continuous annealing process, and asecond annealing process or a plating process. Of these processes, thefirst continuous annealing process which features this method isillustrated in FIG. 9 (in case of a base phase structure being quenchedmartensite) and FIG. 10 (in case of a base phase structure beingquenched bainite).

[0201] First, the hot rolling process and the cooling process arecarried out. Conditions for these processes are not specially limited,but there may be selected suitable working conditions. This is becausein this method (2) it is not that a desired structure is ensured throughthe hot rolling process and the cooling process, but this method ischaracteristic in that the desired structure is obtained by controllingthe subsequent first continuous annealing process and second continuousannealing process or plating process.

[0202] To be more specific, in the hot rolling process there may beadopted for example conditions such that after the end of hot rolling ata temperature of not lower than A_(r3) point, cooling is performed at anaverage cooling rate of about 30° C./s, followed by winding at atemperature of about 500° to 600° C. In the cooling process it isrecommended that cold rolling be carried out at a cooling rate of about30% to 70%. Of course, no limitation is made thereto.

[0203] Next, a description will be given below about the firstcontinuous annealing process {circle over (3)} and the second continuousannealing process or plating process {circle over (4)} as processeswhich feature the method (2).

{circle over (3)} First Continuous Annealing Process (First ContinuousAnnealing Process)

[0204] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₃ point and a step ofcooling the steel sheet to a temperature of not higher than Ms point ora temperature of not lower than Ms point and not higher than Bs point atan average cooling rate of 10° C./s. These conditions have been set forobtaining a desired base phase structure (quenched martensite orquenched bainite).

[0205] First, after soaking to a temperature of not lower than A₃ point(T1 in FIGS. 9 and 10) (preferably 1300° C. or lower), cooling isperformed to a temperature of not higher than Ms point (T2 in FIG. 9) ora temperature of not lower than Ms point and not higher than Bs point(T2 in FIG. 10) while controlling an average cooling rate (CR) to a 20°C./s or higher (preferably 30° C./s or higher), whereby a desiredquenched martensite or quenched bainite is obtained while avoidingferrite transformation or pearlite transformation.

[0206] If the average cooling rate (CR) is lower than the above coolingrate, there will be produced ferrite and pearlite and the desiredstructure will not be obtained. An upper limit of the average coolingrate is not specially limited. The higher, the better. However, it isrecommended to control the upper limit appropriately in relation to theactual operation level.

{circle over (4)} Second Continuous Annealing Process (SubsequentContinuous Annealing Process) or Plating Process

[0207] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point for 10 to 600 seconds, a step of cooling the steel sheetto a temperature of not lower than 300° C. and not higher than 480° C.at an average cooling rate of not lower than 3° C./s, and a step ofholding the steel sheet in the said temperature range from 1 second ormore.

[0208] This process is the same as the continuous annealing process orplating process {circle over (2)} described in the foregoing method (1).This process has been established for tempering the base phase structure(quenched martensite or quenched bainite) produced in the firstcontinuous annealing process {circle over (3)} to obtain not only adesired tempered martensite but also a fine, second phase structure.

[0209] For producing an alloyed, hot dip galvanized steel sheet it isrecommended to adopt the foregoing series of methods (a) to (c). This isbecause the number of “coarse crystal grains” present on the surface ofthe alloyed, hot dip galvanized layer is decreased, so that there isobtained a steel sheet superior also in the sliding property of theplated surface while ensuring the ductility improving effect based onγ_(R). The details thereof will become apparent by reference to theabove methods.

(B) Steel Sheet with a Base Phase Structure Being a Mixed Structure of(Tempered Martensite and Ferrite) or (Tempered Bainite and Ferrite)

[0210] The following methods (3) and (4) are mentioned as typicalmethods for producing this steel sheet.

(3) [Hot Rolling Process]→[Continuous Annealing Process or PlatingProcess]

[0211] This method produces a desired steel sheet through {circle over(1)} a hot rolling process and {circle over (2)} a continuous annealingprocess or a plating process. The hot rolling process {circle over (1)}is illustrated in FIG. 6 in case of a base phase structure comprisingquenched martensite and ferrite and in FIG. 7 in case of a base phasestructure comprising quenched bainite and ferrite. The continuousannealing process or plating process {circle over (2)} is illustrated inFIG. 8.

{circle over (1)} Hot Rolling Process

[0212] The hot rolling process comprises a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C. and a step ofcooling the rolled steel sheet to a temperature of not higher than Mspoint (in case of a base phase structure comprising quenched martensiteand ferrite) or a temperature of not lower than Ms point and not higherthan Bs point (in case of a base phase structure comprising quenchedbainite and ferrite) at an average cooling rate of not lower than 10°C./s and winding up the steel sheet These hot rolling conditions havebeen established for obtaining a desired base phase structure (a mixedstructure of quenched martensite and ferrite or of quenched bainite andferrite), of which the hot rolling finish condition is as described inthe hot rolling process {circle over (1)} in connection with theforegoing method (1).

[0213] The hot rolling finish is followed by cooling. In the methodaccording to the present invention, by controlling the cooling rate(CR), ferrite is partially produced during cooling to provide a twophase region (α+γ), followed by cooling to a temperature of not higherthan Ms point or a temperature of not lower than Ms point and not higherthan Bs point, whereby it is possible to obtain a desired mixedstructure.

[0214] The following methods (a) and (b), preferably (b), are mentionedas methods for the above cooling. (a) A one-step cooling method inwhich, at an average cooling rate of not lower than 10° C./s (preferablynot lower than 20° C./s), cooling is made to a temperature of not higherthan Ms point or a temperature of not lower than Ms point and not higherthan Bspointwhile avoidingpearlitetransformation. At this time, bycontrolling the average cooling rate appropriately, there can beobtained a desired mixed structure (quenched martensite and ferrite, orquenched bainite and ferrite). In the present invention it isrecommended that the content of ferrite be controlled to a value of notlower than 5% and lower than 30% in terms of a space factor relative tothe whole structure. In this case, it is preferred that the averagecooling rate be controlled to 30° C./s or higher.

[0215] The average cooling rate after hot rolling exerts an influence onnot only the formation of ferrite but also the final form of γ_(R). Ifthe average cooling rate is high (preferably 50° C./s or higher), therewill be obtained a lath form. An upper limit of the average cooling rateis not specially limited. The higher, the better. However, in relationto the actual operation level it is recommended to control the upperlimit appropriately.

[0216] Further, for allowing a desired mixed structure to be producedmore efficiently during cooling, it is recommended to adopt (b) atwo-step cooling method which comprises {circle over (1)} a step ofcooling the steel sheet to a temperature in the range of 700±100° C.(preferably 700±50° C.) at an average cooling rate (CR1) of not lowerthan 30° C./s, {circle over (2)} a step of cooling the steel sheet withair in the said temperature range for 1 to 30 seconds, and {circle over(3)} a subsequent step of cooling the steel sheet to a temperature ofnot higher than Ms point or a temperature of not lower than Ms point andnot higher than Bs point at an average cooling rate (CR2) of not lowerthan 30° C./s and winding up the steel sheet. By such stepwise cooling,polygonal ferrite low in dislocation density can be produced in a morepositive manner.

[0217] In both temperature ranges {circle over (1)} and {circle over(3)} it is recommended that cooling be done at an average cooling rateof not lower than 30° C./s, preferably not lower than 40° C./s. An upperlimit of the average cooling rate is not specially limited. The higher,the better. But it is recommended to control the upper limitappropriately in relation to the actual operation level.

[0218] In the above temperature range {circle over (2)} it is preferablethat air cooling be done for 1 second or more, more preferably 3 secondsor more, whereby a predetermined amount of ferrite can be obtainedefficiently. However, if the air cooling time exceeds 30 seconds,ferrite will be produced in an amount exceeding a preferred quantitativerange thereof, resulting in that not only it is impossible to obtain adesired strength, but also the stretch flange formability isdeteriorated. Preferably, the air cooling time is not longer than 20seconds.

[0219] The winding temperature (CT) is as described in the foregoing(1)-{circle over (1)}.

[0220] In the hot rolling process it is recommended that the constituentsteps be controlled appropriately in order to obtain a desired basephase structure. But as to other conditions, including heatingtemperature, there may be adopted conventional conditions (e.g., about1000 to 1300° C.) as necessary.

{circle over (2)} Continuous Annealing Process or Plating Process

[0221] After the hot rolling process {circle over (1)} there isperformed continuous annealing or plating. But if the shape after hotrolling is unsatisfactory, then for the purpose of correcting the shapethere may be performed cooling after the hot rolling {circle over (1)}and before the continuous annealing or plating {circle over (2)}. It isrecommended that the cooling rate be set in the range of 1% to 30%.

[0222] This continuous annealing or plating process comprises a step ofholding the steel sheet in a heated state at a temperature of not lowerthan A₁ point and not higher than A₃ point for 10 to 600 seconds, a stepof cooling the steel sheet to a temperature of not lower than 300° C.and not higher than 480° C. at an average cooling rate of not lower than3° C./s, and a step of holding the steel sheet in the said temperaturerange for 1 second or more. These conditions have been established fortempering the base phase structure produced in the hot rolling processto afford not only a desired mixed structure (tempered martensite andferrite, or tempered bainite and ferrite) but also a desired secondphasestructure. The details thereof are as described above in the continuousannealing process or plating process {circle over (2)} in connectionwith the foregoing method (1).

[0223] The above cooling is followed by austempering, the details ofwhich are as described above in the continuous annealing process orplating process {circle over (2)} in connection with the foregoingmethod (1).

[0224] For producing an alloyed, hot dip galvanized steel sheet it isrecommended to adopt the series of methods (a) to (c) described above.This is because by adopting those methods the number of “coarse grainparticles” present on the surface of the alloyed, hot dip galvanizedlayer is decreased, so that there is obtained a steel sheet superioralso in the sliding property of the plated surface while ensuring theductility improving effect based on γ_(R). The details thereof willbecome apparent by reference to the foregoing method.

(4) [Hot Rolling Process]→[Cold Rolling Process]→[First ContinuousAnnealing Process]→[Second Continuous Annealing Process or PlatingProcess]

[0225] This method (4) produces a desired steel sheet through a hotrolling process, a cooling process, a first continuous annealingprocess, and a second continuous annealing process or a plating process.Of these processes, the first continuous annealing process whichfeatures the method (4) is illustrated in FIG. 11 in case of a basephase structure comprising quenched marternsite and ferrite and in FIG.12 in case of a base phase structure comprising quenched bainite andferrite.

[0226] First, hot rolling and cooling are carried out. These processesare not specially limited. Usually, suitable working conditions may beselected and adopted, the details of which are as described inconnection with the foregoing method (2).

[0227] Next, a description will be given below about {circle over (3)}the first continuous annealing process and {circle over (4)} the secondcontinuous annealing process or the plating process as processes whichfeature the method (4).

{circle over (3)} First Continuous Annealing Process (Initial ContinuousAnnealing Process)

[0228] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point and a step of cooling the steel sheet to a temperature ofnot higher than Ms point (in case of a base phase structure comprisingquenched martensite and ferrite) or a temperature of not lower than Mspoint and not higher than Bs point (in case of a base phase structurecomprising quenched bainite and ferrite) at an average cooling rate ofnot lower than 10° C./s. These conditions have been established forobtaining a desired base phase structure.

[0229] First, soaking is performed to a temperature of not lower than A₁point and not higher than A₃ point (T1 in FIGS. 11 and 12) (preferably1300° or lower). Ferrite is produced partially during soaking if thesoaking temperature is in the range of A₁ to A₃ or during cooling if thesoaking temperature is not lower than A₃ point to provide two phases of[ferrite (α)+γ], followed by cooling to a temperature of not higher thanMs point or a temperature of not lower than Ms point and not higher thanBs point to obtain desired (a +quenched martensite) or (α+quenchedbainite).

[0230] After the above soaking, an average cooling rate (CR) iscontrolled to 10° C./s or higher (preferably 20° C./s or higher) andcooling is allowed to proceed to a temperature of not higher than Mspoint (T2 in FIG. 11) or a temperature of not lower than Ms point andnot higher than Bs point (T2 in FIG. 12) to obtain a desired mixedstructure (quenched martensite and ferrite, or quenched bainite andferrite) while avoiding pearlite transformation. In the presentinvention it is recommended that the content of ferrite be controlled toa value of not less than 5% and less than 30%. In this case, it ispreferable that the average cooling rate be controlled to 30° C./s orhigher.

[0231] The average cooling rate exerts an influence not only on theformation of ferrite but also on the final form of γ_(R), and a highaverage cooling rate (preferably 50° C./s or higher) will result in alath form. An upper limit of the average cooling rate is not speciallylimited. The higher, the better. But it is recommended to control theupper limit appropriately in relation to the actual operation level.

{circle over (4)} Second Continuous Annealing Process (SubsequentContinuous Annealing Process)

[0232] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point for 10 to 600 seconds, a step of cooling the steel sheetto a temperature of not lower than 300° C. and not higher than 480° C.at an average cooling rate of not lower than 3° C./s, and a step ofholding the steel sheet in the said temperature range for 1 second ormore. This process is the same as the second continuous annealingprocess or plating process {circle over (4)} in the foregoing method (2)and has been established for tempering the base phase structure producedin the first continuous annealing process {circle over (3)} to affordnot only a desired structure but also a desired second phase structure.

[0233] For producing an alloyed, hot dip galvanized steel sheet it isrecommended to adopt the foregoing series of methods (a) to (c). Withthose methods, the number of “coarse crystal grains” present on thesurface of the alloyed, hot dip galvanized layer is decreased, so thatthere is obtained a steel sheet superior also in sliding property of theplated surface while ensuring the ductility improving effect based onγ_(R). The details thereof will become apparent by reference to theforegoing methods.

[0234] Next, the following description is provided about the second highstrength steel sheet according to the present invention.

[0235] Having made studies earnestly for producing a low alloyed TRIPsteel sheet having high stretch flange formability and elongation andyet superior in fatigue characteristic, we found out that the expectedobject could be achieved by making control to predetermined base phasestructure and second phase structure described in connection with theabove first high strength steel sheet and by suppressing the formationof a coarse second phase structure, and accomplished the presentinvention on the basis of that finding. More particularly, a mostimportant point of the present invention resides in the finding that theforegoing phase structure containing tempered martensite or temperedbainite is extremely useful in improving the stretch flange formabilityand total elongation and that suppressing the formation of coarsecrystal grains in the second phase structure which contains retainedaustenite is effective in improving the stretch flange formability andfatigue characteristic. With this finding, we were the first to providea low alloyed TRIP steel sheet having both a remarkably improved stretchflange formability and a satisfactory fatigue characteristic whileensuring such a good strength-ductility balance as in the conventionalretained austenite steel sheet.

[0236] A detailed reason why such excellent characteristics are obtainedis not clear, but it is presumed that if there is used as the base phasestructure a soft lath structure containing tempered martensite ortempered bainite, martensite is produced between the laths in the courseof formation of the structure, thus affording a very fine structure,with consequent improvement of stretch flange formability and furtherimprovement of elongation, and that since the method according to thepresent invention includes a step for precipitating a carbide(cementite) between the laths of quenched martensite or quenchedbainite, the formation of a coarse second phase structure is suppressed,resulting in not only the stretch flange formability but also fatiguecharacteristic being improved.

[0237] Reference will first be made below to the second phase structurewhich features the second steel sheet to the greatest extent. The basephase structure in the second steel sheet is the same as that in thefirst steel sheet described above.

[0238] It is necessary for the secondphase structure to satisfy thestructure of the first steel sheet described above and further satisfythe following expression (1):

(S1/S)×100≦20   (1)

[0239] where S stands for a total area of the second phase structure andS1 stands for a total area of coarse second phase structure crystalgrains (Sb) present in the second phase structure, the Sb occupyingthree times or more of an average crystal grain area (Sm) of the secondphase structure.

[0240] The above expression (1) means that the ratio of coarse crystalgrains [those three times or more as large as an average crystal grainarea (Sm) of the second phase structure] to the whole of the secondphase structure which contains retained austenite is to be suppressed to20% or less in terms of an area ratio. With this expression, it isintended to improve fatigue characteristic. According to the results ofour studies it has turned out that the lowering in fatiguecharacteristic of the TRIP steel sheet is attributable to the formationof coarse γ_(R) and the fatigue characteristic is improved if the coarseγ_(R) is diminished and that, for example, such a tempering process aswill be described later [allowing a carbide (cementite) to beprecipitated between laths of the base phase structure] is effective forthat purpose.

[0241] A specific calculating method in connection with the foregoingexpression (1) is as follows.

[0242] First, a steel sheet is subjected to Lepera etching and is thenobserved through an optical microscope (×1000) to provide two picturesof steel sheet structure. Then, an area of 50 μm×50 μm is selected andcut out arbitrarily from each of the photographs. With respect to thetwo pictures thus cut out there are determined a total area of thesecond phase structure (γ_(R), martensite as necessary) relative to thetotal area of the two pictures (50 μm×50 μm×2), as well as an averagecrystal grain area (Sm) of the second phase structure.

[0243] Next, there is calculated a total area of coarse second phasecrystal grains (Sb) present in the second phase structure. To be morespecific, crystal grains having an average area three times as large asthe average crystal grain area (Sm) of the second phase structuredetermined by the above method are defined to be “coarse second phasecrystal grains (Sb),” then the coarse second phase crystal grains (Sb)are totaled and the result is assumed to be a total area (S1) of Sb.

[0244] If (S1/S)×100 is 20 or less, the steel sheet concerned issuperior in fatigue characteristic [fatigue endurance ratio (fatiguestrength σ_(w)/yield strength YP)]. As to the said ratio, the smaller,the better, and it is recommended to control it to 15 or less, morepreferably 10 or less.

[0245] Next, a description will be given below of basic components whichconstitute the second steel sheet. All of the following chemicalcomponents are in mass %.

C: 0.06 to 0.25%

[0246] C is an element essential for ensuring a high strength and forensuring γ_(R). More particularly, C is an element important forensuring a sufficient amount of C in γ phase and for allowing adesiredyphase to remaineven at roomtemperature. However, if C is addedin an amount exceeding 0.25%, the weldability will be deteriorated andcementite will become coarse in a tempering process which will bedescribed later, leading finally to coarsening of the second phasestructure.

[0247] As to the other components than C, they are the same as in thefirst steel sheet described previously.

[0248] How to produce the second steel sheet will be described belowstructure by structure.

(A) Steel Sheet with a Base Phase Structure Being Tempered Martensite orTempered Bainite

[0249] The following methods (5) and (6) are mentioned as typicalmethods for producing the second steel sheet. These methods aresubstantially the same as the method (1) and (2) described previously inconnection with the first steel sheet. A difference resides in that inthe following methods (5) and (6) there is provided a predeterminedtempering process between the hot rolling process and the continuousannealing process or the plating process or between the first continuousannealing process and the second continuous annealing process or theplating process.

[0250] The methods (5) and (6) will be described below in detail.

(5) [Hot Rolling Process]→[Tempering Process]→[Continuous AnnealingProcess or Plating Process]

[0251] This method produces a desired steel sheet through {circle over(1)} hot rolling process, {circle over (2)} tempering process, and{circle over (3)} continuous annealing process or plating process. Ofthese processes, the annealing process {circle over (1)} is illustratedin FIG. 6 (in case of a base phase structure being quenched martensite)and FIG. 7 (in case of a base structure being quenched bainite), and thecontinuous annealing or plating process {circle over (3)} is illustratedin FIG. 8. {circle over (1)} Hot Rolling Process

[0252] The hot rolling process comprises a step of terminating finishrolling at a temperature of not lower than (A_(r33)−50)° C. and a stepof cooling the rolled steel sheet to a temperature of not higher than Mspoint (in case of abase phase structure being tempered martensite) or atemperature of not lower than Ms point and not higher than Bs point (incase of the base phase structure being tempered bainite) at an averagecooling rate of not lower than 20° C./s and winding up the steel sheet.These hot rolling conditions are established for obtaining a desiredbase phase structure (quenched martensite or quenched bainite).

[0253] No matter which base phase structure is to be obtained, it isrecommended that a hot rolling finish temperature (FDT) be set at atemperature of not lower than (A_(r3)−50)° C., preferably not lower thanA_(r3) point. This is for obtaining a desired quenched martensite orquenched bainite in cooperation with subsequent “cooling to atemperature of not higher than Ms point” or “cooling to a temperature ofnot lower than Ms point and not higher than Bs point.”

[0254] The hot rolling described above is followed by cooling. As tocooling conditions (CR), it is recommended that cooling be performed toa temperature of not higher than Ms point while avoiding ferritetransformation and pearlite transformation at an average cooling rate ofnot lower than 20° C./s (preferably not lower than 30° C./s). Thispermits to obtain a desired quenched martensite or quenched bainitewithout formation of polygonal ferrite. The average cooling rate afterthe hot rolling also exerts an influence on the final form of γ_(R). andif the average cooling rate is high, a lath form will result. An upperlimit of the average cooling rate is not specially limited. The higher,the better. But it is recommended to control the upper limitappropriately in relation to the actual operation level.

[0255] For obtaining quenched martensite it is necessary to set thewinding temperature (CT) at a temperature of not higher than Ms point[calculating expression: Ms=561−474×[C]−33×[Mn]−17×[Ni]−17×[Cr]−21×[Mo]where [ ] represents mass % of each element. This is because, if thewinding temperature exceeds Ms point, a desired tempered martensite willnot be obtained and bainite will be formed.

[0256] On the other hand, for obtaining quenched bainite it is necessaryto set the winding temperature (CT) at a temperature of not lower thanMs point and not higher than Bs point [calculating expression: Ms is thesame as in the above expression;Bs=830−270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−80×[Mo] where [ ] represents mass% of each element. This is because, if the winding temperature exceedsBs point, a desired quenched bainite will not be obtained, while if thewinding temperature is lower than Ms point, tempered martensite will beproduced.

[0257] In the hot rolling process, it is recommended to control theabove constituent steps appropriately in order to obtain a desiredquenched martensite or quenched bainite. But as to other conditions,including heating temperature, there may be adopted conventionalconditions (e.g., about 1000 to 1300° C.) suitably.

{circle over (2)} Tempering Process

[0258] The above hot rolling process {circle over (1)} is followed by atempering process. However, if the shape after the hot rolling isunsatisfactory, then for the purpose of correcting the shape, coolingmay be performed after the hot rolling {circle over (1)} and before thetempering {circle over (2)}. In this case, it is recommended to set thecooling rate at 1 to 30%. This is because, if cold rolling is performedat a cooling rate exceeding 30%, the rolling load will increase, makingit difficult to carry out cold rolling.

[0259] The annealing process comprises carrying out tempering at atemperature of not lower than 400° C. and not higher than A_(c1) pointfor a period of time of not shorter than 10 minutes and shorter than 2hours. This tempering process has been established for obtaining adesired γ_(R) (fine γ_(R)) which is effective in improving the fatiguecharacteristic. By going through this tempering process, cementite isprecipitated in the lath boundary of the base phase structure (quenchedmartensite or quenched bainite), and in the subsequent continuousannealing process or plating process {circle over (2)} there is formedafine γ_(R) with the cementite as nucleus, so that itbecomes possible todiminish coarse γ_(R) produced in the pre-austenite grain boundary andblock boundary. Further, there accrues an advantage that, since thestrength of the steel sheet having been subjected to the above temperingprocess decreases, a sheet passing load for passage of the sheet to thesubsequent continuous annealing process {circle over (3)} decreases.

[0260] More specifically, tempering is carried out at a temperature ofnot lower than 400° C. and not higher than A_(c1) point (about 700° C.)for a period of time of not shorter than 10 minutes and shorter than 2hours. This is because if the tempering temperature exceeds thistemperature, there will occur an inverse transformation, preventingsufficient precipitation of cementite. Preferably, the temperingtemperature is not higher than 650° C. On the other hand, the lowerlimit of the tempering temperature has been determined so as to permitcementite to be precipitate as short a time as possible, takingproductivity into account. Preferably, the lower limit is 450° C. Thetempering time is also important for obtaining a desired structure, andif it is shorter than 10 minutes, the precipitation of cementite will beinsufficient. Preferably, the tempering time is 15 minutes or longer. Onthe other hand, if the tempering time is 2 hours or longer, cementitewill become coarse to a remarkable extent, not affording the effect ofmicrostructurization of γ_(R). Preferably, the tempering time is notlonger than 1 hour.

[0261] In case of obtaining a base phase structure of quenched bainiteand if, in the above hot rolling process {circle over (1)}, cooling ismade to a temperature of not lower than 400° C. and not higher thanA_(c1) point at an average cooling rate of not lower than 20° C./s, thetempering process {circle over (2)} is not needed. This is because theforegoing hot rolling process is the same as this tempering process{circle over (1)}. In this case, therefore, the hot rolling process maybe immediately followed by continuous annealing or plating {circle over(3)} which will be described below.

{circle over (3)} Continuous Annealing Process or Plating Process

[0262] The above tempering process {circle over (2)} is followed bycontinuous annealing or plating. This continuous annealing or platingprocess comprises a step of holding the steel sheet in a heated state ata temperature of not lower than A₁ point and not higher than A₃ pointfor 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in this temperature range for 1 second or more. Theseconditions have been established for tempering the base phase structure(quenched martensite or quenched bainite) produced in the hot rollingprocess to obtain not only a desired tempered martensite but also afine, second phase.

[0263] First, soaking is performed at a temperature of not lower than A₁point and not higher than A₃ point (T3 in FIG. 8) for 10 to 600 seconds(t3 in FIG. 8) to produce a desired structure (tempered martensite andγ_(R), or tempered bainite and γ_(R)) (annealing in two phase region).This is because if the soaking temperature exceeds the abovetemperature, the resulting structure will all become γ, while thesoaking temperature is lower than the above temperature, a desired γwill not be obtained. Further, controlling the heating holding time (t3)is particularly important for obtaining a desired structure. This isbecause if the holding time is shorter than 10 seconds, tempering willbe insufficient and a desired base phase structure (tempered martensiteor tempered bainite) will not be obtained. Preferably, the holding timeis not less than 20 seconds, more preferably not less than 30 seconds.If the holding time exceeds 600 seconds, it becomes impossible to retainthe lath structure which is a feature of tempered martensite or temperedbainite, with consequent deterioration of mechanical characteristics.Preferably, the holding time is not more than 500 seconds, morepreferably not more than 400 seconds.

[0264] Next, an average cooling rate (CR) is controlled to a rate of notlower than 3° C./s (preferably not lower than 5° C./s) and cooling ismade to a temperature of not lower than 300° C. (preferably not lowerthan 350° C.) while avoiding pearlite transformation, followed byholding in this temperature range for 1 second or more (preferably 5seconds or more: t4 in FIG. 8) (austempering), whereby the concentrationof C to γ_(R) can be done in a large quantity and in an extremely shorttime.

[0265] If the average cooling rate is lower than the above range, therewill not be obtained a desired structure and pearlite will beproduced.No special limitationisplacedonitsupperlimit. The higher, the better.But it is recommended to control the upper limit appropriately inrelation to the actual operation level.

[0266] Of the above conditions, particularly the austemperingtemperature (T4) is important for ensuring the desired structure andallowing the operation of the present invention to be exhibited. If theaustempering temperature is controlled to the above temperature range,γ_(R) will be obtained stably in a large quantity, whereby there isexhibited TRIP effect based on γ_(R). An austempering temperature oflower than 300° C. will lead to the presence of martensite phase, whilean austempering temperature exceeding 480° C. will result in a largelyincreased amount of bainite phase.

[0267] An upper limit of the holding time (t4) is not specially limited,but when the time taken for transformation of austenite into bainite isconsidered, it is recommended to control the holding time to a time ofnot longer than 3000 seconds, preferably not longer than 2000 seconds.

[0268] In the above process, in addition to the desired base phasestructure (tempered martensite or tempered bainite) and martensite,there may be produced bainite structure insofar as it does not impairthe operation of the present invention. Further, plating and alloyingmay be conducted insofar as the desired structure is not decomposedremarkably nor does the application of plating and alloying impair theoperation of the present invention.

(6) [Hot Rolling Process]→[Cold Rolling Process]→[First ContinuousAnnealing Process]→[Tempering Process]→[Second Continuous AnnealingProcess or Plating Process]

[0269] This method produces a desired steel sheet through a hot rollingprocess, a cold rolling process, a first continuous annealing process, atempering process, and a second annealing process or a plating process.Of these processes, the first annealing process which features thismethod is illustrated in FIG. 9 (in case of a base phase structure beingquenched martensite) and FIG. 10 (in case of a base phase structurebeing quenched bainite).

[0270] First, the hot rolling process and the cooling process arecarried out. These processes are not specially limited, but conventionalconditions may be selected and adopted suitably. This is because in thismethod (6) the hot rolling process and the cooling process are not forensuring a desired structure, but a feature of this method resides incontrolling the subsequent first continuous annealing process, temperingprocess, and second continuous annealing process or plating process toobtain a desired structure.

[0271] More specifically, as conditions for the hot rolling processthere may be adopted such conditions as cooling at an average coolingrate of about 30° C./s after the end of hot rolling conducted at atemperature of not lower than A_(r3) point and winding at a temperatureof about 500° to 600° C. In the cooling process it is recommended toperform cold rolling at a cooling rate of about 30% to 70%. It goeswithout saying that no limitation is made thereto.

[0272] Next, the following description is now provided about the firstcontinuous annealing process {circle over (4)}, the tempering process{circle over (5)}, and the second continuous annealing process orplating process {circle over (6)}, all of which feature this method (6).

{circle over (4)} First Continuous Annealing Process (Initial ContinuousAnnealing Process)

[0273] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₃ point and a step ofcooling the steel sheet to a temperature of not higher than Ms point ora temperature of not lower than Ms point and not higher than Bs point atan average cooling rate of not lower than 10° C./s. These conditionshave been established for obtaining a desired base phase structure(quenched martensite or quenched bainite).

[0274] First, soaking is performed at a temperature of not lower than A₃point (T1 in FIGS. 9 and 10) (preferably not higher than 1300° C.), thenan average cooling rate (CR) is controlled to a temperature of not lowerthan 20° C./s (preferably not lower than 30° C./s) and cooling is madeto a temperature of not higher than Ms point (T2 in FIG. 9) or atemperature of not lower than Ms point and not higher than Bs point (T2in FIG. 10), whereby a desired quenched martensite or quenched bainiteis obtained while avoiding ferrite transformation and pearlitetransformation.

[0275] If the average cooling rate (CR) is lower than the above range,there will be produced ferrite and pearlite and it will be impossible toobtain the desired structure. An upper limit of the average cooling rateis not specially limited. The higher, the better. But it is recommendedto control the upper limit appropriately in relation to the actualoperation level.

{circle over (5)} Tempering Process

[0276] This process is the same as the tempering process {circle over(2)} in the foregoing method (5) and has been established for forming adesired fine γ_(R).

[0277] In the case where a base phase structure of quenched bainite isto be obtained and if, in the first continuous annealing process {circleover (4)}, cooling is performed to a temperature of not lower than 400°C. and not higher than A_(c1) point at an average cooling rate of notlower than 10° C./s, followed by holding at this temperature for notshorter than 10 minutes and shorter than 2 hours, this tempering process{circle over (5)} becomes unnecessary. This is because the abovecontinuous annealing process is the same as the tempering process{circle over (5)}. In this case, the foregoing continuous annealingprocess may be immediately followed by the second continuous annealingor plating {circle over (6)} which will be described below.

{circle over (6)} Second Continuous Annealing Process (SubsequentContinuous Annealing Process) or Plating Process

[0278] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point for 10 to 600 seconds, a step of cooling the steel sheetto a temperature of not lower than 300° C. and not higher than 480° C.at an average cooling rate of not lower than 3° C./s, and a step ofholding the steel sheet in this temperature range for 1 second or more.

[0279] This process is the same as the continuous annealing process orplating process {circle over (3)} in the foregoing method {circle over(5)} and has been established for tempering the base phase structure(quenched martensite or quenched bainite) produced in the firstcontinuous annealing process {circle over (4)} to obtain not only adesired tempered martensite but also a desired fine, second phasestructure.

(B) Steel Sheet with a Base Phase Structure Being a Mixed Structure of(Tempered Martensite and Ferrite) or (Tempered Bainite and Ferrite)

[0280] The following methods (7) and (8) are mentioned as typicalmethods for producing the second steel sheet according to the presentinvention. These methods are substantially the same as the foregoingmethods (3) and (4) described in connection with the first steel sheet.A difference resides in that in these methods a predetermined temperingprocess is provided between the hot rolling process and the continuousannealing process or the plating process or between the first continuousannealing process and the second continuous annealing process or theplating process in the methods (3) and (4).

(7) [Hot Rolling Process]→[Tempering Process]→[Continuous AnnealingProcess or Plating Process]

[0281] This method produces a desired steel sheet through {circle over(1)} a hot rolling process, {circle over (2)} a tempering process, and{circle over (3)} a continuous annealing process or a plating process.Of these processes, the hot rolling process {circle over (1)} isillustrated in FIG. 6 in case of a base phase structure comprisingquenched martensite and ferrite and in FIG. 7 in case of a base phasestructure comprising quenched bainite and ferrite, and the continuousannealing or plating process {circle over (3)} is illustrated in FIG. 8.

{circle over (1)} Hot Rolling Process

[0282] The hot rolling process comprises a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C. and a step ofcooling the rolled steel sheet to a temperature of not higher than Mspoint (in case of a base phase structure comprising quenched martensiteand ferrite) or a temperature of not lower than Ms point and not higherthan Bs point (in case of a base phase structure comprising quenchedbainite and ferrite) at an average cooling rate of not lower than 10°C./s and winding up the steel sheet. These hot rolling conditions havebeen established for obtaining a desired base phase structure (a mixedstructure of quenched martensite and ferrite or quenched bainite andferrite). Of these conditions, the hot rolling finish condition is asdescribed in the hot rolling process {circle over (1)} in connectionwith the foregoing method (5).

[0283] Cooling is performed after the above hot rolling finish.According to the present invention, by controlling the cooling rate(CR), ferrite is partially produced during cooling to provide a twophase region of (α+γ), and by cooling to a temperature of not higherthan Ms point or a temperature of not lower than Ms point and not higherthan Bs point there can be obtained a desired mixed structure.

[0284] The following methods (a) and (b) are mentioned as methods forthe aforesaid cooling.

(a) One-Step Cooling

[0285] At an average cooling rate of not lower than 10° C./s (preferablynot lower than 20° C./s) there is made cooling to a temperature of nothigher than Ms point or a temperature of not lower than Ms point and nothigher than Bs point while avoiding pearlite transformation. At thistime, by controlling the average cooling rate appropriately it ispossible to obtain a desired mixed structure (quenchedmartensite+ferrite, or quenched bainite+ferrite). In the presentinvention it is recommended to control the ferrite content to not lessthan 5% and less than 30% in terms of a space factor relative to thewhole structure. In this case, it is recommended to control the averagecooling rate to 30° C./s or higher.

[0286] The average cooling rate after hot rolling exerts an influencenot only on the formation of ferrite but also on the final form ofγ_(R), and if the average cooling rate is high (preferably 50° C./s orhigher), a lath form will result. An upper limit of the average coolingrate is not specially limited. The higher, the better. But it isrecommended to control the upper limit appropriately in relation to theactual operation level.

[0287] Further, for producing the desired mixed structure moreefficiently during cooling, it is recommended to adopt (b) a two-stepcooling method which comprises {circle over (1)} a step of cooling thesteel sheet to a temperature in the range of 700±100° C. (preferably700±50° C.) at an average cooling rate (CR1) of not lower than 30° C./s,{circle over (2)} a step of conducting air cooling in the saidtemperature range for 1 to 30 seconds, and {circle over (3)} a step ofsubsequently cooling the steel sheet to a temperature of not higher thanMs point or a temperature of not lower than Ms point and not higher thanBs point at an average cooling rate (CR2) of not lower than 30° C./s andwinding up the steel sheet. By thus cooling stepwise, polygonal ferritelow in dislocation density can be produced more positively.

[0288] In the temperature ranges {circle over (1)} and {circle over (3)}it is recommended that cooling be done at an average cooling rate of notlower than 30° C./s, preferably not lower than 40° C./s. An upper limitof the average cooling rate is not specially limited. The higher, thebetter. But it is recommended to control the upper limit appropriatelyin relation to the actual operation level.

[0289] In the temperature range {circle over (2)} it is preferable thatair cooling be done for 1 second or more, more preferably 3 seconds ormore, whereby a predetermined ferrite quantity is attained efficiently.However, if the air cooling time exceeds 30 seconds, ferrite will beproduced in an amount exceeding the preferred range, with the resultthat a desired strength is not attained and the stretch flangeformability is deteriorated. Preferably, the air cooling time is notlonger than 20 seconds.

[0290] The winding temperature (CT) is as described in the hot rollingprocess {circle over (1)} in connection with the foregoing method (5).

[0291] In the hot rolling process it is recommended to control each ofthe constituent steps appropriately in order to obtain a desired basephase structure. As to other conditions, including heating temperature,conventional conditions (e.g., about 1000 to 1300° C.) may be selectedsuitably.

{circle over (2)} Tempering Process

[0292] The hot rolling {circle over (1)} described above is followed bytempering. However, if the shape after the hot rolling isunsatisfactory, then for the purpose of correcting the shape there maybe performed cooling after the hot rolling {circle over (1)} and beforethe tempering {circle over (2)}. In this case, it is recommended to setthe cold rolling rate at 1 to 30%.

[0293] The tempering process has been established for obtaining adesired fine γ_(R) and the details thereof are as described in thetempering process {circle over (2)} in connection with the foregoingmethod (5).

[0294] In the case where a mixed base phase structure of quenchedbainite and ferrite is to be obtained and if, in the hot rolling process{circle over (1)}, cooling is made to a temperature of not lower than400° C. and not higher than A_(c1) point at a predetermined averagecooling rate and is followed by holding at this temperature for a periodof time of not shorter than 10 minutes and shorter than 2 hours, thetempering process {circle over (2)} becomes unnecessary. This is becausethe above hot rolling process is the same as this tempering process{circle over (2)}. In this case, the above hot rolling process maybeimmediately followed by {circle over (3)} continuous annealing orplating which will be described later.

{circle over (3)} Continuous Annealing Process or Plating Process

[0295] The above tempering process {circle over (2)} is followed bycontinuous annealing or plating. The continuous annealing or platingprocess comprises a step of holding the steel sheet in a heated state ata temperature of not lower than A₁ point and not higher than A₃ pointfor 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in this temperature range for 1 second or more. Theseconditions have been established for tempering the base phase structureproduced in the hot rolling process to obtain not only a desired mixedstructure (tempered martensite+ferrite, or tempered bainite+ferrite) butalso a fine, second phase structure. The details thereof are asdescribed in the continuous annealing process or plating process {circleover (3)} in connection with the foregoing method (5).

[0296] For producing a desired amount of Cγ more efficiently duringcooling it is recommended to adopt, for the above cooling step, atwo-step cooling method comprising {circle over (1)} a step of coolingthe steel sheet to a temperature (Tq) of (A₁ point to 600° C.) at anaverage cooling rate of not higher than 15° C./s and {circle over (2)} astep of cooling the steel sheet to a temperature of not lower than 300°C. and not higher than 480° C. at an average cooling rate of not lowerthan 20° C./s.

[0297] If cooling is made to the above temperature range {circle over(1)} at an average cooling rate of not higher than 15° C./s (preferablynot higher than 10° C./s), ferrite is the first to be produced and Ccontained in the ferrite is concentrated to γ. If cooling issubsequently performed to the above temperature range {circle over (2)}at an average cooling rate of not lower than 20° C./s (preferably notlower than 30° C./s, more preferably not lower than 40° C./s), thetransformation of γ into pearlite is suppressed and γ remains even at alow temperature, thus affording the desired γ_(R) structure.

[0298] An upper limit of the average cooling rate is not speciallylimited. The higher, the better. But it is recommended to control theupper limit appropriately in relation to the actual operation level.

[0299] The above cooling process is followed by austempering, thedetails of which are as described in the continuous annealing or platingprocess {circle over (3)} in connection with the foregoing method (5).

(8) [Hot Rolling Process]→[Cold Rolling Process]→[First ContinuousAnnealing Process]→[Tempering Process]→[Second Continuous AnnealingProcess or Plating Process]

[0300] This method (8) produces a desired steel sheet through a hotrolling process, a cooling process, a first continuous annealingprocess, a tempering process, and a second continuous annealing processor a plating process. Of these processes, the first continuous annealingprocess which features the method (8) is illustrated in FIG. 11 in caseof a base phase structure comprising quenched martensite and ferrite andin FIG. 12 in case of a base phase structure comprising quenched bainiteand ferrite.

[0301] First, the hot rolling process and the cooling process areexecuted. These processes are not specially limited. Usually, suitableworking conditions may be selected and adopted, the details of which areas described in the foregoing method (6).

[0302] A description will be given below about {circle over (4)} thefirst continuous annealing process, {circle over (5)} the temperingprocess, and {circle over (6)} the second continuous annealing process,all of which feature the above method (8).

{circle over (4)} First Continuous Annealing Process (Initial ContinuousAnnealing Process)

[0303] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point and a step of cooling the steel sheet to a temperature ofnot higher than Ms point (in case of a base phase structure comprisingquenched martensite and ferrite) or a temperature of not lower than Mspoint and not higher than Bs point (in case of a base phase structurecomprising quenched bainite and ferrite) at an average cooling rate ofnot lower than 10° C./s. These conditions have been established forobtaining a desired base phase structure.

[0304] First, soaking is performed at a temperature of not lower than A₁point and not higher than A₃ point (T1 in FIGS. 11 and 12) (preferably1300° C. or higher). If soaking is conducted at a temperature of A1 toA3, ferrite is partially produced during soaking, while if soaking isconducted at a temperature of not lower than A₃ point, ferrite ispartially produced during cooling, to provide two phases of [ferrite(α)+γ], followed by cooling to a temperature of not higher than Ms pointor a temperature of not lower than Ms point and not higher than Bs pointto obtain desired (α+quenched martensite) or (α+quenched bainite).

[0305] After the above soaking step, an average cooling rate (CR) iscontrolled to a rate of not lower than 10° C./s (preferably not lowerthan 20° C./s) and cooling is performed to a temperature of not higherthan Ms point (T2 in FIG. 11) or a temperature of not lower than Mspoint and not higher than Bs point (T2 in FIG. 12) to afford a desiredmixed structure (quenched martensite+ferrite, or quenchedbainite+ferrite) while avoiding pearlite transformation. In the presentinvention it is recommended to control the ferrite content to a value ofnot less than 5% and less than 30%. In this case, it is preferable thatthe average cooling rate be controlled to 30° C./s or higher.

[0306] The average cooling rate exerts not only on the formation offerrite but also on the final form of γ_(R). and if the average coolingrate is high (preferably 50° C./s or higher), a lath form will result.An upper limit of the average cooling rate is not specially limited. Thehigher, the better. But it is recommended to control the upper limitappropriately in relation to the actual operation level.

{circle over (5)} Tempering Process

[0307] This process has been established for obtaining a desired fineγ_(R) and the details of tempering conditions are as described in thetempering process {circle over (5)} in connection with the foregoingmethod (6).

[0308] In the case where a mixed base phase structure of quenchedbainite and ferrite is to be obtained and if, in the above continuousannealing process {circle over (4)}, cooling is performed to atemperature of not lower than 400° C. and not higher than A_(c1) pointat an average cooling rate of not lower than 10° C./s, followed byholding at this temperature for not shorter than 10 minutes and shorterthan 2 hours, the tempering process {circle over (5)} becomesunnecessary. This is because the foregoing first continuous annealingprocess is the same as the tempering process {circle over (5)}. In thiscase, the first continuous annealing process may be immediately followedby the second continuous annealing or plating process {circle over (6)}which will be described below.

{circle over (6)} Second Continuous Annealing Process (SubsequentContinuous Annealing Process) or Plating Process

[0309] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point for 10 to 600 seconds, a step of cooling the steel sheetto a temperature of not lower than 300° C. and not higher than 480° C.at an average cooling rate of not lower than 3° C./s, and a step ofholding the steel sheet in this temperature range for 1 second or more.This process is the same as the second continuous annealing or platingprocess Ax in the foregoing method (6) and has been established fortempering the base phase structure produced in the foregoing firstcontinuous annealing process {circle over (4)} to obtain not only adesired structure but also a fine, second phase structure.

[0310] Lastly, reference will be made below to the foregoing third highstrength steel sheet.

[0311] We have made earnest studies for providing a low alloy TRIP steelsheet having high stretch flange formability and elongation and superiorin bake hardening (BH) property, especially a TRIP steel sheet capableof exhibiting an excellent bake hardening property even in a very largestrain-loaded area such as an area where suspension members are mounted.As a result we found out the following points and accomplished thepresent invention.

[0312] (1) If control is made so that {circle over (1)} a temperedmartensite structure, {circle over (2)} a mixed structure of temperedmartensite and ferrite, {circle over (3)} a tempered bainite structure,and {circle over (4)} a mixed structure of tempered bainite and ferrite,as soft lath structures low in dislocation density, are each produced asa base phase structure and a structure having a retained austenite(γ_(R)) phase is produced as a second phase structure, there is obtaineda high strength steel sheet which satisfies the condition of BH(2%)≧70MPa due to an excellent bake hardening property which each of thosestructures possesses.

[0313] (2) In addition to the above structure control, if retainedaustenite as a second phase structure is dispersed uniformly and finely(shortening the diffusion distance up to dislocation) in pre-austenitegrain boundaries, as well as block and packet boundaries, in the(quenched martensite or quenched bainite) structure prior to tempering,there is obtained a high strength steel sheet which further satisfiesthe following condition:

BH(10%)≧BH(2%)/2

[0314] and a very excellent bake hardening property can be ensured evenin a very large strain area.

[0315] (3) Such a finely dispersed, retained austenite as referred toabove can be obtained by controlling the heating temperature (SRT)within a low range before hot rolling to a rather low temperature,allowing rolling to proceed in the austenite region.

[0316] In the present invention, a detailed reason why BH property isimproved, especially why excellent BH property is obtained even in alarge strain area, by more finely dispersing the retained austenite asthe second phase, is not clear, but is presumed to be as follows. Asnoted previously, BH property is obtained by an interaction (fixing ofdislocation by C) between dislocation and solid solution C, butparticularly in a large strain area there occurs a phenomenon that solidsolution C is insufficient, although sufficient dislocation is obtained.However, when the retained austenite as the second phase, which is asupply source of solid solution C, is finely dispersed, the diffusiondistance up to dislocation becomes shorter, so that a decrease of BHquantity due to the lack of solid solution C can be prevented. This ispresumed to be the reason why an extremely excellent BH property isexhibited.

[0317] In connection with the mechanism of “BH property” it is presumedthat dislocation which has been introduced into the base phase byworking is fixed to C (solid solution C) in steel by heat treatmentafter working, giving rise to hardening, resulting in an increase oftensile yield stress.

[0318] Description is now directed to “BH (2%) quantity” as referred toherein. When a tensile test piece (usually a JIS No.5 test piece) ispulled up to 2% in terms of a nominal strain, a deformation stress σ1 ismeasured, then after the removal of load, the test piece is held at 170°C. for 20 minutes, then tensile test is again conducted and an upperyield stress σ2 (a stress corresponding to 0.2% proof stress in the casewhere a yield point does not appear) is measured. The BH (2%) quantityin question is represented by the difference between σ1 and σ2. (In theworking Examples to be described later it will be referred to as BH2.)

[0319] “BH (10%) quantity” as referred to herein is measured in the sameway as is the case with the above BH (2%) quantity except that in theabove measurement of BH (2%) quantity a tensile test piece (usually aJIS No.5 test piece) is pulled up to 10% in terms of a nominal strainand the resulting deformation stress is measured. In the workingExamples to be described later it will be referred to as BH10.

[0320] Thus, the BH (2%) quantity defines BH property in an ordinarystrain region, while the BH (10%) defines BH property in a large strainregion.

[0321] The steel sheet according to the present invention satisfies thecondition that the BH (2%) quantity should be not less than 70 MPa(preferably not less than 80 MPa, more preferably not less than 90 MPa)and that the BH (10%) quantity should be not less than half of the BH(2%) quantity, (not less than 35 MPa), preferably not less than 40 MPa,more preferably not less than 45 MPa.

[0322] As to the base phase structure and the second phase structureboth featuring the above steel sheet, they are as described above inconnection with the first steep sheet.

[0323] A description will be given below about basic components whichconstitute the above third steel sheet. All of the following chemicalcomponents are in mass %.

C: 0.06 to 0.25%

[0324] C is an element for ensuring a high strength and for ensuringγ_(R). More particularly, C is an element important for providing asufficient content of C in γ phase and for allowing a desired γ phase toremain even at room temperature. However, if C is added in an amountexceeding 0.25%, the weldability will be deteriorated.

[0325] Other components than the above C are as described above inconnection with the first steel sheet.

[0326] How to produce the third steel sheet will be described belowstructure by structure.

(A) Steel Sheet with a Base Phase Structure Being Tempered Martensite orTempered Bainite

[0327] The following methods (9) and (10) are mentioned as typicalmethods for producing the third steel sheet. These methods are the sameas the methods (1) and (2) which have been described above in connectionwith the first steel sheet except that in these methods the heatingtemperature (SRT) prior to hot rolling in the methods (3) and (4) iscontrolled to a temperature of 950 to 1100° C.

[0328] A detailed description will be given below about each of themethods.

(9) [Hot Rolling Process]→[Continuous Annealing Process or PlatingProcess]

[0329] This method produces a desired steel sheet through {circle over(1)} a hot rolling process and {circle over (2)} a continuous annealingprocess or a plating process. The hot rolling process {circle over (2)}is illustrated in FIG. 6 (in case of a base phase structure beingquenched martensite) and in FIG. 7 (in case of a base phase structurebeing quenched bainite) and the continuous annealing process or platingprocess {circle over (2)} is illustrated in FIG. 8.

{circle over (1)} Hot Rolling Process

[0330] This process comprises a step of controlling a heatingtemperature (SRT) before hot rolling to a temperature of 950° to 1100°C. and terminating finish rolling at a temperature of not lower than(A_(r3)−50)° C. and a step of cooling the resulting steel sheet to atemperature of not higher than Ms point (in case of a base phasestructure being tempered martensite) or. a temperature of not lower thanMs point and not higher than Bs point (in case of a base phase structurebeing tempered bainite) at an average cooling rate of not lower than 20°C./s and winding up the steel sheet. These hot rolling conditions(especially SRT condition) have been established for obtaining a desiredbase phase structure (quenched martensite or quenched bainite beforetempering), also for making the pre-austenite grain diameter fine in(during) hot rolling, and for reducing the block and packet size as amore specific structure size relative to the pre-austenite graindiameter in the (quenched martensite or quenched bainite) structure, tothereby disperse γ_(R) of the second phase structure finely anduniformly in the pre-austenite grain region and block and packetboundaries.

[0331] First, the heating temperature (SRT) before hot rolling iscontrolled to a temperature of 950° to 1100° C. and a hot rolling finishtemperature (FDT) is set at a temperature of not lower than (A_(r3)−50)°C.

[0332] The control of the heating temperature (SRT) before hot rollingis extremely important for obtaining a desired second phase structure(finely dispersed γ_(R)) and it is not until controlling the heatingtemperature to a temperature in the range of 950° to 1100° C. that theabove structure can be obtained. A heating temperature lower than 950°C. substantially overlaps the hot rolling finish temperature (FDT) whichwill be described later. On the other hand, if the heating temperatureexceeds 1100° C., it will become impossible to obtain the desired BHproperty [especially BH (10%)]. Preferably, the heating temperature isnot lower than 975° C. and not higher than 1075° C.

[0333] In the present invention the SRT is controlled lower than that inthe conventional TRIP steel sheet. In the conventional steel sheet theSRT is controlled generally to the range of 1100° C. exclusive to 1300°C. inclusive. However, we have confirmed experimentally that in thistemperature range the desired finely dispersed, retained austenite phaseis not obtained and an excellent bake hardening property cannot beensured particularly in a large strain region (see the working Examplesto be described later).

[0334] Controlling the hot rolling finish temperature (FDT) is importantfor obtaining a desired quenched martensite or quenched bainite incooperation with “cooling to a temperature of not higher than Ms point”or “cooling to a temperature of not lower than Ms point and not higherthan Bs point” which follows the finish rolling. It is recommended tocontrol the FDT to a temperature of not lower than (A_(r3)−50)° C.,preferably not lower than A_(r3) point. Like the foregoing SRT, the FDTplays an important role also for obtaining a desired second structure,so in addition to the foregoing control of SRT, if FDT is controlled toa temperature of not lower than (A_(r3)−50)° C. and not higher thanA_(r3) point, a desired second phase can be obtained more efficiently.That is, by controlling both SRT and FDT to lower values than those forthe conventional steel sheet it is possible to ensure an extremelysuperior BH property.

[0335] The above hot rolling process is followed by cooling. It isrecommended that cooling be performed at an average cooling rate (CR) ofnot lower than 20° C./s (preferably not lower than 30° C./s) to atemperature of not higher than Ms point while avoiding ferritetransformation and pearlite transformation. With this cooling, a desiredquenched martensite or quenched bainite can be obtained. The averagecooling rate after hot rolling exerts an influence also on the finalform of γ_(R), and if the average cooling rate is high, a lath form willresult. An upper limit of the average cooling rate is not speciallylimited, and the higher, the better. But it is recommended to controlthe upper limit appropriately in relation to the actual operation level.

[0336] For obtaining quenched martensite it is necessary that thewinding temperature (CT) be not higher than Ms point [calculatingexpression: Ms=561−474×[C]−33×[Mn]−17×[Ni]−17×(Cr]−21×[Mo] where [ ]represents mass % of each element]. This is because if the windingtemperature exceeds Ms point, a desired quenched martensite is notobtained and there are formed bainite, etc.

[0337] On the other hand, for obtaining quenched bainite it is necessaryto set the winding temperature (CT) at a temperature of not lower thanMs point and not higher than Bs point [calculating expression: Ms is thesame as the above expression;Bs=830−270×[C]−90×[Mn]−37×[Ni]−70×[Cr]−80×[Mo] where [ ] represents mass% of each element]. This is because if the winding temperature exceedsBs point, a desired quenched bainite is not obtained, while if it islower than Ms point, quenched martensite is produced.

[0338] In the hot rolling process it is recommended to control each ofthe above constituent steps appropriately in order to obtain desiredquenched martensite or quenched bainite. But as to other conditions,including heating temperature, conventional conditions (e.g. about 1000to 1300° C.) may be selected suitably.

{circle over (2)} Continuous Annealing Process or Plating Process

[0339] The above hot rolling process {circle over (1)} is followed bycontinuous annealing or plating. However, if the shape after hot rollingis unsatisfactory, then for the purpose of correcting the shape, coolingmay be done after the hot rolling {circle over (1)} and before thecontinuous annealing or plating. In this case it is recommended to setthe cold rolling rate at 1 to 30%. This is because if cold rolling iscarried out at a cooling rate exceeding 30%, the rolling load willincrease, making it difficult to effect cold rolling.

[0340] The continuous annealing process or plating process comprises astep of holding the steel sheet in a heated state at a temperature ofnot lower than A₁ point and not higher than A₃ point for 10 to 600seconds, a step of cooling the steel sheet to a temperature of not lowerthan 300° C. and not higher than 480° C. at an average cooling rate ofnot lower than 3° C./s and a step of holding the steel sheet in thetemperature range for 1 second or more. These conditions have beenestablished for tempering the base phase structure (quenched martensiteor quenched bainite) produced in the hot rolling process to afford notonly a desired tempered martensite but also a fine, second phase (γ_(R))

[0341] First, soaking is performed at a temperature of not lower than A₁point and not higher than A₃ point (T3 in FIG. 8) for 10 to 600 seconds(t3 in FIG. 8) to produce a desired structure (tempered martensite andγ_(R), or tempered bainite and γ_(R)) (annealing in two phase region).This is because if the soaking temperature exceeds the abovetemperature, the resulting structure will also become γ, while if it islower than the above temperature, the desired γ_(R) will not beobtained. Further, controlling the heating holding time (t3) isparticularly important for obtaining the desired structure. This isbecause if the holding time is shorter than 10 seconds, tempering willbe insufficient, not affording the desiredbase phase structure (temperedmartensite or tempered bainite). Preferably the holding time is 20seconds or longer, more preferably 30 seconds or longer. If the holdingtime exceeds 600 seconds, it becomes impossible to retain the lathstructure which is a feature of tempered martensite or tempered bainite,and mechanical properties are deteriorated. Preferably the holding timeis not longer than 500 seconds, more preferably not longer than 400seconds.

[0342] Next, the average cooling rate (CR) is controlled to a rate ofnot lower than 3° C./s (preferably not lower than 5° C./s) and coolingis performed to a temperature (bainite transformation: T4 in FIG. 4) ofnot lower than 300° C. (preferably not lower than 350° C.) and nothigher than 480° C. (preferably not higher than 450° C.) while avoidingpearlite transformation, followed by holding in this temperature rangefor 1 second or more (preferably 5seconds or more: t4 in FIG. 8)(austempering), whereby the concentration of C to γ_(R) can be obtainedin a large quantity and in an extremely short time.

[0343] If the average cooling rate is lower than the above range, thedesired structure will not be obtained, with formation of pearlite, etc.An upper limit of the average cooling rate is not specially limited, andthe higher, the better. But it is recommended to control the upper limitappropriately in relation to the actual operation level.

[0344] For producing a desired amount of Cγ more efficiently duringcooling, it is recommended that the above cooling step be carried out bya two-step cooling method which comprises {circle over (1)} a step ofcooling the steel sheet.up to a temperature (Tq) of (A₁ point to 600°C.) at an average cooling rate of not higher than 15° C./s and {circleover (2)} a step of cooling the steel sheet to a temperature of notlower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 20° C./s.

[0345] If cooling is performed to the above temperature range {circleover (1)} at an average cooling rate of not higher than 15° C./s(preferably not higher than 10° C./s), C will be concentrated in alarger quantity to γ. If cooling is subsequently performed to the abovetemperature range {circle over (2)} at an average cooling rate of notlower than 20° C./s (preferably not lower than 30° C./s, more preferablynot lower than 40° C./s), the transformation of γ into pearlite issuppressed and there remains γ even at a low temperature, resulting inthat the desired γ_(R) structure is obtained. An upper limit of theaverage cooling rate is not specially limited. The higher, the better.But it is recommended to control the upper limit appropriately inrelation to the actual operation level.

[0346] The cooling described above is followed by austempering. Theaustempering temperature (T4) is important for ensuring the desiredstructure and allowing the operation of the present invention to beexhibited. If control is made to the foregoing temperature range, astable and large amount of γ_(R) will be obtained, whereby TRIP effectbased on γ_(R) is exhibited. If the austempering temperature is lowerthan 300° C., martensite phase will exist, while if it exceeds 480° C.,bainite phase will increase in a large amount.

[0347] An upper limit of the holding time (t4) is not specially limited,but when the time taken for transformation of austenite into bainite isconsidered, it is recommended to control the holding time to a time ofnot longer than 3000 seconds, preferably not longer than 2000 seconds.

[0348] In the above process, in addition to the desired base phasestructure (tempered martensite or tempered bainite) and martensite therealso maybe produced bainite structure insofar as the operation of thepresent invention is not impaired. Further, plating and alloying may beperformed insofar as the desired structure is not decomposed markedlynor is impaired the operation of the present invention.

(10) [Hot Rolling Process]→[Cold Rolling Process]→[First ContinuousAnnealing Process]→[Second Continuous Annealing Process or Platingprocess]

[0349] This method (10) produces a desired steel sheet through a hotrolling process, a cold rolling process, a first continuous annealingprocess, and a second continuous annealing process or plating process.Of these processes, the first continuous annealing process whichfeatures this method is illustrated in FIG. 9 (in case of a base phasestructure being quenched martensite) and in FIG. 10 (in case of a basephase structure being quenched bainite).

[0350] First, the hot rolling process and the cold rolling process arecarried out. As noted earlier, controlling the heating temperature (SRT)before hot rolling is extremely important for obtaining a desired secondphase structure (finely dispersed γ_(R)) It is not until controlling theheating temperature to a temperature in the range of 950° to 1100° C.that the desired structure can be obtained. If the heating temperatureis lower than 950° C., it substantially overlaps a hot rolling finishtemperature (FDT) which will be described later, while if it exceeds1100° C., a desired BH property [especially BH (10%)] is not attained.Preferably the heating temperature in question is not lower than 975° C.and not higher than 1075° C.

[0351] In the present invention, the SRT is controlled to a lowertemperature than in the conventional TRIP sheet. In the conventionalsteel sheet, the SRT is controlled generally to a temperature in therange of 1100° C. exclusive to 1300° C. inclusive. However, we haveconfirmed experimentally that with such a temperature range, a desired,finely dispersed, retained austenite phase is not obtained and that itis impossible to ensure an excellent bake hardening propertyparticularly in a large strain region (see the working Examples to bedescribed later).

[0352] Other hot rolling and cold rolling conditions are not speciallylimited, but there may be adopted conventional conditions. To be morespecific, for the above hot rolling process there may be adopted suchconditions as, after the end of hot rolling at a temperature of notlower than A_(r3) point, cooling the steel sheet at an average coolingrate of about 30° C./s and winding up the steel sheet at a temperatureof about 500° to 600° C. In the cold rolling process it is recommendedto carry out cold rolling at a cold rolling rate of about 30% to 70%. Ofcourse, no limitation is made thereto.

[0353] Next, reference will be made below to {circle over (3)} the firstcontinuous annealing process and {circle over (4)} the second continuousannealing process or plating process, both featuring the method (10).

{circle over (3)} First Continuous Annealing Process (Initial ContinuousAnnealing Process)

[0354] This process comprises a step of holding the steel sheet at atemperature of not lower than A₃ point and a step of cooling the steelsheet to a temperature of not higher than Ms point or a temperature ofnot lower than Ms point and not higher than Bs point at an averagecooling rate of not lower than 10° C./s. These conditions have beenestablished for obtaining a desired structure (quenched martensite orquenched bainite).

[0355] First, soaking is performed at a temperature of not lower than A₃point (T1 in FIGS. 9 and 10) (preferably not higher than 1300° C.), thenan average cooling rate (CR) is controlled to a rate of not lower than20° C./s (preferably not lower than 30° C./s) and cooling is performedto a temperature of not higher than Ms point (T2 in FIG. 9) or atemperature of not lower than Ms point and not higher than Bs point (T2in FIG. 10), whereby desired quenched martensite or quenched bainite isobtained while avoiding ferrite transformation and pearlitetransformation.

[0356] If the average cooling rate (CR) is lower than the above range,ferrite and pearlite will be produced and the desired structure will notbe obtained. An upper limit of the average cooling rate (CR) is notspecially limited. The higher, the better. But it is recommended tocontrol the upper limit appropriately in relation to the actualoperation level.

{circle over (4)} Second Continuous Annealing Process (SubsequentContinuous Annealing Process) or Plating Process

[0357] This process comprises a step of holding the steel sheet at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C., and a step ofholding the steel sheet in this temperature range for 1 second or more.

[0358] This process is the same as the continuous annealing process orplating process {circle over (3)} in the foregoing method (9) and hasbeen established for tempering the base phase structure (quenchedmartensite or quenched bainite) produced in the first continuousannealing process {circle over (4)} to obtain not only a desiredtempered martensite but also a fine, second phase-structure.

(B) Steel Sheet with a Base Phase Structure Being a Mixed Structure of(Tempered Martensite and Ferrite) or (Tempered Bainite and Ferrite)

[0359] The following methods (11) and (12) are mentioned as typicalmethods for producing this steel sheet. These methods are the same asthe methods (3) and (4) which have been described above in connectionwith the first steel sheet except that in these methods the heatingtemperature (SRT) before hot rolling in the methods (3) and (4) iscontrolled to a temperature of 950° to 1100° C.

(11) [Hot Rolling Process]→[Continuous Annealing Process or PlatingProcess]

[0360] This method produces a desired steel sheet through {circle over(1)} a hot rolling process and {circle over (2)} a continuous annealingprocess or a plating process. The hot rolling process {circle over (1)}is illustrated in FIG. 6 in case of a base phase structure comprisingquenched martensite and ferrite and in FIG. 7 in case of a base phasestructure being quenched bainite and ferrite. The continuous annealingprocess or plating process {circle over (2)} is illustrated in FIG. 8.

{circle over (1)} Hot Rolling Process

[0361] The hot rolling process comprises a step of controlling theheating temperature (SRT) before hot rolling to a temperature in therange of 950° to 1100° C., a step of terminating finish rolling at atemperature of not lower than (A_(r3)−50)° C., and a step of cooling thesheet to a temperature of not higher than Ms point (in case of a basephase structure comprising quenched martensite and ferrite) or atemperature of not lower than Ms point and not higher than Bs point (incase of a base phase structure comprising quenched bainite and ferrite)at an average cooling rate of not lower than 10° C./s and winding up thesteel sheet. These hot rolling conditions have been established forobtaining a desired base phase structure (quenched martensite andferrite, or quenched bainite and ferrite) and a second phase structure.Of these conditions, hot rolling start and finish conditions are asdescribed in the hot rolling process {circle over (1)} in the foregoingmethod (9).

[0362] The hot rolling finish step is followed by cooling. According tothis method, by controlling the cooling rate (CR), ferrite is partiallyproduced during cooling to provide a two-phase region (α+γ), and bycooling to a temperature of not higher than Ms point or a temperature ofnot lower than Ms point and not higher than Bs. point it is possible toobtain a desired mixed structure.

[0363] For effecting the above cooling step there may be adopted thefollowing method (a) or (b), preferably (b).

[0364] (a) A one-step cooling method involving cooling the steel sheetat an average cooling rate of not lower than 10° C./s (preferably notlower than 20° C./s) to a temperature of not higher than Ms point or atemperature of not lower than Ms point and not higher than Bs pointwhile avoiding pearlite transformation. At this time, by controlling theaverage cooling rate appropriately it is possible to obtain a desiredmixed structure (quenched martensite+ferrite, or quenchedbainite+ferrite). In the present invention it is recommended to controlthe ferrite content to not less than 5% and less than 30% in terms of aspace factor relative to the whole structure. In this case it ispreferable to control the average cooling rate to a rate of not lowerthan 30° C./s.

[0365] The average cooling rate after hot rolling exerts an influencenot only on the formation of ferrite but also on the final form ofγ_(R), and if the average cooling rate is high (preferably 50° C./s orhigher), a lath form will result. An upper limit of the average coolingrate is not specially limited. The higher, the better. But it isrecommended to control the upper limit appropriately in relation to theactual operation level.

[0366] Further, for producing the desired mixed structure moreefficiently during cooling, it is recommended to adopt (b) a two-stepcooling method which comprises {circle over (1)} a step of cooling thesteel sheet to a temperature in the range of 700±100° C. (preferably700±50° C.) at an average cooling rate (CR1) of not lower than 30° C./s,{circle over (2)} a step of cooling the steel sheet with air in the saidtemperature range for 1 to 30 seconds, and {circle over (3)} a step ofsubsequently cooling the steel sheet to a temperature of not higher thanMs point or a temperature of not lower than Ms point and not higher thanBs point at an average cooling rate (CR2) of not lower than 30° C./s andwinding up the steel sheet. By such stepwise cooling, polygonal ferritelow in dislocation density can be produced more positively.

[0367] In both the temperature ranges in the above steps {circle over(1)} and {circle over (3)} it is recommended to conduct cooling at anaverage cooling rate of not lower than 30° C./s, preferably not lowerthan 40° C./s. An upper limit of the average cooling rate is notspecially limited, and the higher, the better. But it is recommended tocontrol the upper limit appropriately in relation to the actualoperation level.

[0368] In the temperature range in the above step {circle over (2)} itis preferable that air cooling be done for 1 second or more, preferably3 seconds or more, whereby a predetermined ferrite quantity is obtainedefficiently. However, if the air cooling time exceeds 30 seconds, theferrite quantity will exceed a preferred range, making it impossible toattain a desired strength and leading to deterioration of the stretchflange formability. Preferably, the air cooling time is not longer than20 seconds.

[0369] The winding temperature (CT) is as described in the rollingprocess {circle over (1)} in the foregoing method (9).

[0370] In the hot rolling process it is recommended to control each ofthe above constituent steps appropriately in order to obtain a desiredbase phase structure. But as to other conditions, including heatingtemperature, conventional conditions (e.g., about 1000 to 1300° C.) maybe selected suitably.

{circle over (2)} Continuous Annealing Process or Plating Process

[0371] The above hot rolling process {circle over (1)} is followed bycontinuous annealing or plating. But if the shape after hot rolling isunsatisfactory, then for the purpose of correcting the shape, coolingmay be performed after the hot rolling {circle over (1)} and before thecontinuous annealing or plating {circle over (2)}. In this case, it isrecommended that the cooling be done at a cold rolling rate of 1 to 30%.

[0372] The continuous annealing or plating process comprises a step ofholding the steel sheet in a heated state at a temperature of not lowerthan A₁ point and not higher than A₃ point for 10 to 600 seconds, a stepof cooling the steel sheet to a temperature of not lower than 300° C.and not higher than 480° C. at an average cooling rate of not lower than3° C./s, and a step of holding the steel sheet in the said temperaturerange for 1 second or more. These conditions have been established fortempering the base phase structure produced in the hot rolling processto afford not only a desired mixed structure (temperedmartensite+ferrite, or tempered bainite+ferrite) but also a fine, secondphase structure. The details thereof are as described in the continuousannealing process or plating process {circle over (3)} in connectionwith the foregoing method (1).

[0373] For producing a desired amount of Cγ more efficiently duringcooling it is recommended that the above cooling step be carried out bya two-step cooling method which comprises {circle over (1)} a step ofcooling the steel sheet to a temperature (Tq) of (A₁ point to 600° C.)at an average cooling rate of not higher than 15° C./s and {circle over(2)} a step of cooling the steel sheet to a temperature of not lowerthan 300° C. and not higher than 480° C. at an average cooling rate ofnot lower than 20° C./s.

[0374] If cooling is made to the temperature range in the above step{circle over (1)} at an average cooling rate of not higher than 15° C./s(preferable not higher than 10° C./s), first ferrite is produced and Ccontained in the ferrite is concentrated into γ. Subsequently, ifcooling is conducted to the temperature range in the above step {circleover (2)} at an average cooling rate of not lower than 20° C./s(preferably not lower than 30° C./s, more preferably not lower than 40°C./s), the transformation of γ into pearlite is suppressed and γ remainseven at a low temperature, resulting in that the desired γ_(R) structureis obtained. An upper limit of the average cooling rate is not speciallylimited. The higher, the better. But it is recommended to control theupper limit appropriately in relation to the actual operation level.

[0375] The above cooling step is followed by austempering, the detailsof which are as described in the continuous annealing or plating process{circle over (2)} in connection with the foregoing method (9).

(12) [Hot Rolling Process]→[Cold Rolling Process]→[First ContinuousAnnealing Process]→[Tempering Process]→[Second annealing process orPlating process]

[0376] This method (12) produces a desired steel sheet through a hotrolling process, a cold rolling process, a first continuous annealingprocess, a tempering process, and a second annealing process or aplating process. Of these processes, the first continuous annealingprocess, which features this method (12) is illustrated in FIG. 11 incase of a base phase structure comprising quenched martensite andferrite and in FIG. 12 in case of a base phase structure comprisingquenched bainite and ferrite.

[0377] First, the hot rolling process and the cold rolling process arecarried out. The details of these processes are as described in theforegoing method (10).

[0378] A description will be given below about the first continuousannealing process {circle over (3)} and the second continuous annealingprocess or plating process {circle over (4)}, both featuring the method(12).

{circle over (3)} First Continuous Annealing Process (Initial ContinuousAnnealing Process)

[0379] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point and a step of cooling the steel sheet to a temperature ofnot higher than Ms point (in case of a base phase structure comprisingquenched martensite and ferrite) or a temperature of not lower than Mspoint and not higher than Bs point (in case of a base phase structurecomprising quenched bainite and ferrite) at an average cooling rate ofnot lower than 10° C./s. These conditions have been established forobtaining a desired base phase structure.

[0380] First, soaking is performed at a temperature of not lower than A₁point and not higher than A₃ point (T1 in FIGS. 11 and 12) (preferably1300° C. or lower). Ferrite is partially produced during soaking whensoaking is done at a temperature of A₁ to A₃ or during cooling whensoaking is done at a temperature of not lower than A₃ point, to providetwo phases of [ferrite (α)+γ], followed by cooling to a temperature ofnot higher than Ms point or a temperature of not lower than Ms point andnot higher than Bs point, whereby there is obtained desired (α+quenchedmartensite) or (α+quenched bainite).

[0381] After the above soaking step, an average cooling rate (CR) iscontrolled to a rate of not lower than 10° C./s (preferably not lowerthan 20° C./s) and cooling is performed to a temperature of not higherthan Ms point (T2 in FIG. 11) or a temperature of not lower than Mspoint and not higher than Bs point (T2 in FIG. 12), whereby a desiredmixed structure (quenched martensite+ferrite, or quenchedbainite+ferrite) while avoiding pearlite transformation. In the presentinvention it is recommended to control the ferrite content to a value ofnot less than 5% and less than 30%. In this case, it is preferable tocontrol the average cooling rate to 30° C./s or higher.

[0382] The average cooling rate exerts an influence not only on theformation of ferrite but also on the final form of γ_(R), and if theaverage cooling rate is high (preferably 50° C./s or higher), a lathform will result. An upper limit of the average cooling rate is notspecially limited. The higher, the better. But it is recommended tocontrol the upper limit appropriately in relation to the actualoperation level.

{circle over (4)} Second Continuous Annealing Process (SubsequentContinuous Annealing Process) or Plating Process

[0383] This process comprises a step of holding the steel sheet in aheated state at a temperature of not lower than A₁ point and not higherthan A₃ point for 10 to 600 seconds, a step of cooling the steel sheetto a temperature of not lower than 300° C. and not higher than 480° C.at an average cooling rate of not lower than 3° C./s, and a step ofholding the steel sheet in this temperature range for 1 second or more.This process is the same as the second continuous annealing process orplating process {circle over (6)} in the foregoing method (2) and hasbeen established for tempering the base phase structure produced in thefirst continuous annealing process {circle over (3)} to afford not onlya desired structure but also a fine, second phase structure.

[0384] The present invention will be described in detail below by way ofworking Examples thereof. It is to be understood that the followingExamples do not restrict the present invention and that changes andexecution thereof within a scope not departing from the above andlater-described gists of the invention are all included in the technicalscope of the invention.

EXAMPLES Example 1 A Study (Part 1) of Components Compositions in theFirst High Strength Steel Sheet (Base Phase Structure: TemperedMartensite)

[0385] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tolow C steels having a C content of not higher than 0.25% [steels high instrength (TS)×stretch flange formability (λ) and taking weldability intoaccount]. More specifically, steel samples comprising components'compositions described intablel (unit in the table is mass %) werevacuum-melted into slabs for experiment and thereafter hot-rolled steelsheets having a thickness of 2.0 mm were produced in accordance with theforegoing method (1) (hot rolling→continuous annealing).

[0386] More particularly, each of the slabs was heated at 1150° C. for30 minutes, then the finish temperature (FDT) was set at 900° C. andcooling was performed to room temperature at an average cooling rate of50° C./s (hot rolling process), followed by annealing in two phaseregion for 120 seconds, subsequent cooling to 400° C. at an averagecooling rate of 30° C./s, and holding at this temperature for 30 seconds(austempering). These conditions were used as basic conditions.

[0387] Steel sheets thus produced were then measured for tensilestrength (TS), elongation [total elongation (El)], yield strength (YP),and stretch flange formability (hole expanding property: λ), in thefollowing manner.

[0388] In a tensile test, using a JIS No.5 test piece, there weremeasured tensile strength (TS), elongation (El), and yield strength(YP). A strain rate in the tensile test was set at 1 mm/sec.

[0389] In a stretch flange formability test there was used a disc-liketest piece having a diameter of 100 mm and a thickness of 2.0 mm. Morespecifically, a hole 10 mm in diameter was formed by punching and wassubjected to a hole expanding work on burr with a 60° conical punch,then a hole expanding rate (λ) upon crack penetration was measured(Japan Steel Federation JFST 1001).

[0390] As to an area fraction of structure in each of the above steelsheets, each steel sheet was subjected to Lepera etching, then thestructure thereof was identified by observation under a transmissionelectron microscope (TEM; magnification 15000×), and thereafter a spacefactor of the structure was measured by observation through an opticalmicroscope (magnification 1000×). The space factor of γ_(R) and theconcentration of C in γ_(R) were measured by an X-ray diffraction methodafter chemical polishing, following grinding the steel sheet to aquarter thickness thereof (ISIJ Int. Vol.33.(193,3), No.7, P.776).

[0391] The results obtained are shown in Table 2.

See Tables 1, 2

[0392] The following can be seen from the results thus obtained (all ofthe following No. mean Run No. in Table 2).

[0393] First, No. 2 to 5 and 7 to 15, which satisfy the componentsspecified in the present invention, afforded steel sheets ofsatisfactory characteristics.

[0394] For reference, a TEM photograph (magnification: 15000×) of asteel sheet (No. 3) according to the present invention is shown in FIG.13. From this photograph it is seen that the steel sheet according tothe present invention has tempered martensite of a clear lath structure.

[0395] In contrast therewith, the following steel sheets not satisfyingany of the components specified in the present invention have thefollowing disadvantages.

[0396] First, No.1, which is an example of a small amount of C, is lowin both TS and El because desired tempered martensite and γ_(R) are notobtained.

[0397] No. 6, which is an example of a small total amount of (Si+Al) anda small amount of Mn, is as low as 20% in El because a desired γ_(R) isnot obtained.

[0398] For reference, results of characteristic evaluation onconventional TRIP steel sheets are shown in Table 3. Of these steelsheets, No.1 is a DP steel sheet of ferrite and martensite using No. 2steel sample shown in Table 1, No. 2 is a TRIP steel sheet using No. 3steel sample in Table 1 and with polygonal ferrite as a base phase, andNo. 3 is a two phase steel sheet of ferrite and bainite, using No. 2steel sample shown in Table

See Tables 3

[0399] Reference to Table 3 shows that No. 1 is deteriorated in bothelongation and stretch flange formability, No. 2 is deteriorated instretch flange formability, and No. 3 is deteriorated in elongation.

Example 2 A Study (part 2) of Components' Compositions in the First HighStrength Steel Sheet (Base Phase Structure: Tempered Martensite)

[0400] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tohigh C steels having a C content of 0.25 to 0.6% [steels high instrength (TS)×stretch flange formability (λ) and also high inTS×elongation (El)]. More specifically, steel samples comprisingcomponents' compositions shown in Table 4 (unit in the table is mass %)were vacuum-melted, then hot rolled steel sheets were produced in thesame way as in Example 1 and were evaluated for characteristics.

[0401] The results obtained are shown in Table 5.

See Tables 4, 5

[0402] The following can be seen from these results (all of thefollowing No. mean Run No. in Table 5).

[0403] First, all of No. 3 to 6, 8 to 15 and 17, which satisfy thecomposition of a high C steel specified in the present invention,afforded steel sheets of satisfactory characteristics.

[0404] For reference, a TEM photograph (magnification: 15000×) of asteel sheet (No. 3) according to the present invention is shown in FIG.14. From this photograph it is seen that the steel sheet according tothe present invention has tempered martensite of a clear lath structure.

[0405] On the other hand, No. 1 and 2 are low in El because their Ccontents, which are 0.15% and 0.20%, are smaller than in the otherexamples (all being not less than 0.4% in the amount of C).

[0406] No. 7, which is an example of a small amount of Mn and a smalltotal amount of (Si+Al), is as low as 20% in El because a desired γ_(R)is not obtained.

[0407] No. 16 is an example of having produced a large amount ofpearlite structure as a second phase structure due to adoption of asomewhat low cooling rate, in which both El and λ are low.

[0408] For reference, Table 6 shows the results of having evaluatedcharacteristics of a conventional TRIP steel sheet using No. 3 steelsample shown in Table 1 and with polygonal ferrite as a base phase.

See Table 6

[0409] From Table 6 it is seen that the conventional steel sheet is highin El but low in λ.

[0410] Example 3

A Study of Manufacturing Conditions for the First High Strength SteelSheet (Base Phase Structure: Tempered Martensite)

[0411] In this Example, various manufacturing conditions shown in Tables7 and 8 were adopted using No. 3 and No. 4 slabs for experiment shown inTables land 4, respectively. The thickness of each hot rolled steelsheet was set at 2.0 mm and with this as a base there were conductedexperiments.

[0412] Next, the structure of each of the steel sheets was checked inthe same way as in Example 1. The results obtained are also shown inTables 7 and 8. The steels used in this Example are different in onlythe amount of C (C of No. 3 in Table 1 is 0.15% and that of No. 4 inTable 4 is 0.48%) but are substantially the same in the contents ofother components, so that all of the structures obtained were the same.

See Tables 7, 8

[0413] No. 1 to 24 in Table 7 were produced by the foregoing method (1).More specifically, No. 1 to 23 were subjected to hot rolling→continuousannealing and No. 24 was subjected to hot rolling→plating (further,alloying)

[0414] In Table 7, No. 1, 3, 6, 9 to 11, 13, 14, 16, 18, 19, and 22 to24 are examples of production carried out using conditions specified inthe present invention, in which desired structures were obtained.

[0415] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 24 in Table 7 was used andheat-treated under the conditions shown in the same table with theproviso that pre-plating was applied thereto, to afford an alloyed, hotdip galvanized steel sheet. More specifically, after hot rolling hadbeen conducted under the conditions shown in Table 7, Fe pre-plating wasconducted under the following conditions (amount of Fe pre-platingdeposited: 4.0 g/m², amount of hot dip Zn plating: 52 g/m²), followed byplating [plating bath: Zn-010% Al (effective Al concentration), bathtemperature: 460° C.] and subsequent alloying (alloying temperature:450° C., alloying time: 45 seconds).

Fe Pre-Plating Conditions

[0416] Plating bath: FeSO₄.7H₂O (400 g/L)

[0417] Liquid pH: 2.0

[0418] Liquid temp.: 60° C.

[0419] Current density: 50 A/dm²

[0420] As is the case with the omission of pre-plating, the alloyed, hotdip galvanized steel sheet thus Fe pre-plated afforded a satisfactorystructure and was extremely superior in plating characteristics (notshown in the table) such as excellent sliding property and powderingresistance of the plated surface without the lack of plating.

[0421] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0422] No. 2 is an example of a low hot rolling finish temperature(FDT), inwhicha desired structure was not obtained, but ferritestructure was produced.

[0423] No. 4 is an example of a low average cooling rate (CR) in hotrolling, in which ferrite and pearlite were produced.

[0424] No. 5 is an example of a high winding temperature (CT) in hotrolling, in which bainite was produced in a large quantity.

[0425] No. 7 is an example of using a conventional TRIP steel (with abase phase being polygonal ferrite), in which a desired structure wasnot obtained.

[0426] No. 8 is an example of ahigh two phase region temperature (T3) incontinuous annealing, in which a desired structure was not obtained, butbainite structure was obtained as a base phase structure.

[0427] No. 12 is an example of a low T3, in which γ_(R) structure wasnot obtained.

[0428] No. 15 is an example of a short holding time (t3) at a two phaseregion temperature in continuous annealing, in which tempering wasinsufficient and a desired tempered martensite was not obtained.

[0429] No. 17 is an example of a low average cooling rate (CR) incontinuous annealing, in which pearlite was produced.

[0430] No. 20 and 21 are examples low in austempering temperature (T4)(i.e., austempering is not performed), in which a desired structure wasnot obtained, but martensite was produced.

[0431] Next, No. 25 to 27 in Table 7 are examples in which cold rollingwas performed in the foregoing method (1). More specifically, No. 25 and26 are examples of having gone through hot rolling→coldrolling→continuous annealing and No. 27 is an example having gonethrough hot rolling→cold rolling→plating (further, alloying).

[0432] In No. 25 and 27, conditions specified in the present inventionwere adopted to afford desired structures.

[0433] On the other hand, in No. 26 there was adopted a high coolingrate and a desired tempered martensite was not obtained, with formationof polygonal ferrite.

[0434] Lastly, No. 28 to 52 in Table 8 followed the foregoing method(2). More specifically, No. 28 to 51 have gone through hot rolling→coldrolling→first continuous annealing→second continuous annealing, whileNo. 52 has gone through hot rolling→cold rolling→first continuousannealing→plating (further, alloying).

[0435] In No. 28, 31,32, 34, 36 to 38, 41 to 42, 44, 46 to 47, and 50 to52 in Table 8 there were adopted conditions specified in the presentinvention to afford desired structures.

[0436] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 52 in Table 8 was subjected to Fepre-plating and alloying under the same conditions as No. 24. The thusFe pre-plated, alloyed, hot dip galvanized steel sheet proved to have agood structure equal to that obtained without going through pre-plating,and also proved to have extremely superior plating characteristics (notshown in the table) such as superior sliding property and powderingresistance of the plated surface without the lack of plating.

[0437] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0438] No. 29 and 30 are examples of low γ region temperatures (T1) inthe first continuous annealing process, in which ferrite was produced.

[0439] No. 33 is an example of a low average cooling rate (CR) in thefirst continuous annealing process, in which polygonal ferrite andpearlite were produced.

[0440] No. 35 is an example of a high two phase region temperature (T3)in the second continuous annealing process, in which bainite structurewas obtained as a base phase structure.

[0441] No. 39 is an example of a low T3, in which a desired γ_(R)structure was not obtained.

[0442] No. 40 is an example of a long holding time (t3) in a two phasetemperature region in the second continuous annealing process, in whichferrite structure was obtained as a base phase structure.

[0443] No. 43 is an example of a short t3, in which tempering wasinsufficient and a desired tempered martensite was not obtained.

[0444] No. 45 is an example of a low average cooling rate (CR) in thesecond continuous annealing process, in which pearlite was produced.

[0445] No. 48 and 49 are examples low in austempering temperature (T4)(i.e., austempering is not performed), in which martensite was producedand a desired structure was not obtained.

Example 4 A Study (Part 1) of Components' Compositions in the First HighStrength Steel (Base Phase Structure: Tempered Bainite)

[0446] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tolow C steels having a C content of 0.25% or less [steels high instrength (TS)×stretch flange formability (λ) and taking weldability intoaccount]. More specifically, steel samples comprising components'compositions shown in Table 1 (unit in the table is mass %) werevacuum-melted into slabs for experiment, followed by the same procedureas in Example 1 in accordance with the foregoing method (1) (hotrolling→continuous annealing) to afford hot rolled steel sheets eachhaving a thickness of 2.0 mm.

[0447] Then, in the same way as in Example 1 the steel sheets thusobtained were measured for tensile strength (TS), elongation [totalelongation (El)], yield strength (YP), and stretch flange formability(hole expanding property: λ), and in each of the steel sheets there weremeasured an area fraction of structure, a space factor of γ_(R), and theconcentration of C in γ_(R).

[0448] The results obtained are shown in Table 9.

See Table 9

[0449] The following can be seen from these results (all of thefollowing No. mean Run NO. in Table 9).

[0450] First, all of No. 2 to 5 and 7 to 15, which satisfy thecomponents specified in the present invention, afforded steel sheets ofgood characteristics.

[0451] For reference, a TEM photograph (magnification: 15000×) of asteel sheet (No. 3) according to the present invention is shown in FIG.15. From this picture it is seen that the steel sheet according to thepresent invention has tempered bainite of a clear lath structure.

[0452] In contrast therewith, the following examples lacking in any ofthe components specified in the present invention have the followingdisadvantages.

[0453] First, No. 1 is an example of a small amount of C, in which TSand El are low because desired tempered bainite and γ_(R) are notobtained.

[0454] No. 6 is an example of a small total amount of (Si+Al) and asmall amount of Mn, in which El is as low as 10% because a desired γ_(R)is not obtained.

Example 5 A Study (Part 2) of Components' Compositions in the First HighStrength Steel (Base Phase Structure: Tempered Bainite)

[0455] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tohigh C steels having a C content of 0.25 to 0.6% [steels high instrength (TS)×stretch flange formability (λ) and also high inTS×elongation (El)]. More specifically, steel samples comprisingcomponents' compositions shown in Table 4 (unit in the table is mass %)were vacuum-melted and hot rolled steel sheets were produced in the sameway as in Example 1 and then evaluated for various characteristics.

[0456] The results obtained are shown in Table 10.

See Table 10

[0457] The following can be seen from these results (all of thefollowing No. mean Run No. in Table 10).

[0458] First, all of No. 3 to 6, 8 to 15, and 17, which satisfy thecomposition of a high C steel specified in the present invention,afforded steel sheets of good characteristics.

[0459] For reference, a TEM photograph (magnification: 15000×) of asteel sheet (No. 3) according to the present invention is shown in FIG.16. From this photograph it is seen that the steel sheet according tothe present invention has tempered bainite of a clear lath structure.

[0460] On the other hand, No. 1 and 2 are low in El because their Cquantities are smaller than in the other examples (all being not lessthan 0.4% in the amount of C).

[0461] No. 7 is an example of a small amount of Mn and a small totalamount of (Si+Al), in which El is as low as 12% because a desired γ_(R)is not obtained.

[0462] No. 16 is an example of having produced a large amount ofpearlite structure as a second phase structure due to adoption of asomewhat low cooling rate, in which both El and λ are low.

[0463] For reference, Table 11 shows the results of having evaluatedcharacteristics of a conventional TRIP steel sheet using No. 3 steelsample shown in Table 1 and with polygonal ferrite as a base phase.

See Table 11

[0464] From Table 11 it is seen that the conventional steel sheet ishigh in El but low in λ.

Example 6 A Study of Manufacturing Conditions for the First HighStrength Steel Sheet (Base Phase Structure: Tempered Bainite)

[0465] In this Example, various manufacturing conditions shown in Tables12 and 13 were adopted using No. 3 and No. 4 slabs for experiment shownin Tables 1 and 4, respectively (thickness of each hot rolled steelsheet was set at 2.0 mm).

[0466] Next, the structure of each of the steel sheets was checked inthe same way as in Example 1. The results obtained are as shown inTables 12 and 13. The steels used in this Example are different in onlythe amount of C (C of No. 3 in Table 1 is 0.15% and that of No. 3 inTable 4 is 0.41%) but are substantially the same in the contents ofother components, so that all of the structures obtained were the same.

See Tables 12, 13

[0467] First, No. 1 to 23 were produced by the foregoing method (1).More specifically, No. 1 to 22 were subjected to hot rolling→continuousannealing and No. 23 was subjected to hot rolling→plating (further,alloying)

[0468] No. 1, 3, 8 to 10, 12, 13, 15, 17, 18, and 21 to 23 are examplesof production carried out using conditions specified in the presentinvention, in which desired structures were obtained.

[0469] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 23 in Table 12 was used andheat-treated under the conditions shown in Table 12 with the provisothat pre-plating was applied thereto, to afford an alloyed, hot dipgalvanized steel sheet. The details of the pre-plating are as describedin Example 3.

[0470] As is the case with the omission of pre-plating, the alloyed, hotdip galvanized steel sheet thus Fe pre-plated afforded a satisfactorystructure and was extremely superior in plating characteristics (notshown in the table) such as excellent sliding property and powderingresistance of the plated surface without the lack of plating.

[0471] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0472] No. 2 is an example of a low hot rolling finish temperature(FDT), in whichadesired structure was not obtained, but ferritestructure was produced.

[0473] No. 4 is an example of a low average cooling rate (CR) in hotrolling, in which ferrite and pearlite were produced.

[0474] No. 5 is an example of a low winding temperature (CT) in hotrolling, in which tempered martensite was produced.

[0475] No. 6 is an example of a high CT, in which a desired structurewas not obtained, but there was obtained the same structure as that of aconventional TRIP steel (with a base phase being polygonal ferrite).

[0476] No.7 is an example of a high two phase region temperature (T3) incontinuous annealing, in which a desired structure was not obtained, butbainite structure was obtained as a base phase structure.

[0477] No. 11 is an example of a low T3, in which a retained austenite(γ_(R)) structure was not obtained.

[0478] No. 14 is an example of a short holding time (t3) at a two phaseregion temperature in continuous annealing, in which tempering wasinsufficient and a desired tempered bainite was not obtained.

[0479] No. 16 is an example of a low average cooling rate (CR) incontinuous annealing, in which pearlite was produced.

[0480] No. 19 and 20 are examples low in austempering temperature (T4)(i.e., austempering is not performed), in which a desired structure wasnot obtained, but martensite was produced.

[0481] Next, No. 24 to 26 in Table 12 are examples in which cold rollingwas performed in the foregoing method (1). More specifically, No. 24 and25 are examples of having gone through hot rolling→coldrolling→continuous annealing and No. 26 is an example having gonethrough hot rolling→cold rolling→plating (further, alloying).

[0482] In No. 24 and 26, conditions specified in the present inventionwere adopted to afford desired structures.

[0483] On the other hand, in No. 25 there was adopted a high coldrolling rate and a desired tempered bainite was not obtained, withformation of polygonal ferrite.

[0484] Lastly, No. 27 to 51 in Table 13 followed the foregoing method(2). More specifically, No. 27 to 50 have gone through hot rolling→coldrolling→first continuous annealing→second continuous annealing, and No.51 have gone through hot rolling→cold rolling→first continuousannealing→plating (further, alloying).

[0485] In No. 27, 30, 31, 33, 35 to 37, 40 to 41, 43, 45 to 46, and 49to 51 there were adopted conditions specified in the present inventionto afford desired structures.

[0486] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 51 in Table 13 was subjected toFe pre-plating and alloying under the same conditions as No.23 in Table7. The thus Fepre-plated, alloyed, hot dip galvanized steel sheet provedto have a good structure equal to that obtained without going throughpre-plating, and also proved to have extremely superior platingcharacteristics (not shown in the table) such as superior slidingproperty and powdering resistance of the plated surface without the lackof plating.

[0487] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0488] No. 28 and 29 are examples of low γ region temperatures (T1) inthe first continuous annealing process, in which ferrite was produced.

[0489] No. 32 is an example of a low average cooling rate (CR) in thefirst continuous annealing process, in which polygonal ferrite andpearlite were produced.

[0490] No. 34 is an example of a high two phase region temperature (T3)in the second continuous annealing process, in which all of thestructure obtained was not a tempered bainite structure, but was anordinary bainite structure.

[0491] No. 38 is an example of a low T3, in which a desired γ_(R) wasnot obtained.

[0492] No. 39 is an example of a long holding time t3) in a two phasetemperature region in the second continuous annealing process, in whichferrite structure was obtained as a base phase structure.

[0493] No. 42 is an example of a short t3, in which tempering wasinsufficient and a desired tempered bainite was not obtained.

[0494] No. 44 is an example of a low average cooling rate (CR) in thesecond continuous annealing process, in which pearlite was produced.

[0495] No. 47 and 48 are examples low in austempering temperature (T4)(i.e., austempering is not performed), in which martensite was producedand a desired structure was not obtained.

Example 7 A Study (Part 1) of Components' Compositions in the First HighStrength Steel Sheet (Base Phase Structure: a Mixed Structure ofTempered Martensite and Ferrite)

[0496] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tolow C steels having a C content of 0.25% or less [steels high instrength (TS)×stretch flange formability (λ) and taking weldability intoaccount]. More specifically, steel samples comprising components'compositions shown in Table 1 (unit in the table is mass %) werevacuum-melted into slabs for experiment and thereafter the procedure ofExample 1 was repeated in accordance with the foregoing method (3) (hotrolling→continuous annealing) to afford hot rolled steel sheets having athickness of 2.0 mm.

[0497] Then in the same manner as in Example 1 the steel sheets thusobtained were measured for tensile strength (TS), elongation [totalelongation (El)], yield strength (YP), and stretch flange formability(hole expanding property: λ), and also there were measured an areafraction of structure in each of the steel sheets, a space factor ofγ_(R), and the concentration of C in γ_(R).

[0498] The results obtained are shown in Table 14.

See Table 14

[0499] The following can be seen from these results (all of thefollowing No. mean Run No. in Table 14).

[0500] First, all of No. 3 to 6, 8 to 18, and 20, which satisfy theconditions specified in the present invention, afforded steel sheets ofgood characteristics.

[0501] For reference, an optical microphotograph (magnification: 1000×)of a steel sheet (No. 3) according to the present invention is shown inFIG. 17. From this photograph it is seen that the steel sheet accordingto the present invention has tempered martensite of a lath structure.

[0502] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0503] First, No. 1 is an example of a small amount of C, in which γ_(R)was not obtained and it was impossible to ensure a desired El.

[0504] No. 2 is an example of a Cγ_(R) content of less than 0.8% , inwhich it was impossible to ensure a desired El.

[0505] No. 7 is an example of a small amount of Mn and a small totalamount of (Si+Al), in which a desired γ_(R) was not obtained and henceEl was low.

[0506] No. 19 is an example of having adopted a somewhat low coolingrate and a consequent large proportion of pearlite structure, in which apredetermined tempered martensite was not obtained and both El and λwere deteriorated.

Example 8 A Study (Part 2) of Components' Compositions in the First HighStrength Steel Sheet (Base Phase Structure: a Mixed Structure ofTempered Martensite and Ferrite)

[0507] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tohigh C steels having a C content of 0.25 to 0.6% [steels high instrength (TS)×stretch flange formability (λ) and also high inTS×elongation (El)]. More specifically, steel samples comprisingcomponents' compositions described in Table 15 (unit in the table ismass %) were vacuum-melted, then hot rolled steel sheets were producedin the same way as in Example 1 and were evaluated for characteristics.

[0508] The results obtained are shown in Table 16.

See Tables 15, 16

[0509] The following can be seen from these results (all of thefollowing No. mean Run No. in Table 16).

[0510] First, all of No. 4 to 7, 9 to 19, and 21, which satisfy theconditions specified in the present invention, afforded steel sheets ofgood characteristics.

[0511] For reference, an optical microphotograph (magnification: 1000×)of a steel sheet (No. 3) according to the present invention is shown inFIG. 18. From this photograph it is seen that the steel sheet accordingto the present invention has tempered martensite of a lath structure.

[0512] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0513] First, No. 1 is smaller in the amount of C, which is 0.15% , thanin the other examples (C: 0.4% or more) and is low in El.

[0514] No. 2 is also small in the amount of C, which is 0.15% , and isless than 0.8% in the amount of Cγ_(R), low in El.

[0515] No. 3 is less than 0.8% in the amount of Cγ_(R) and it wasimpossible to ensure a desired El.

[0516] No. 8 is an example of a small amount of Mn and a small totalamount of (Si+Al), in which El is low because a desired γ_(R) is notobtained.

[0517] No. 20 is an example of a large proportion of pearlite structurebecause of adoption of a somewhat low cooling rate, in which apredetermined tempered martensite was not obtained and both El and λwere deteriorated.

[0518] For reference, the results of having evaluated characteristics ofconventional TRIP steel sheets are shown in Table 17. In the same table,No. 22 is a DP steel plate of ferrite and martensite using the steelsample of No. 3 in Table 1, No. 23 is a conventional TRIP steel sheetusing the steel sample of No. 3 in Table 1 and with a base phase beingpolygonal ferrite, and No. 24 is a conventional two phase steel sheet offerrite and bainite using the steel sample of No. 3 in Table 1.

See Table 17

[0519] From Table 17 it is seen that No. 22 is deteriorated inelongation and stretch flange formability, No. 23 is deteriorated instretch flange formability, and No.24 is deteriorated in elongation.

Example 9 A Study of Manufacturing Conditions for the First HighStrength Steel Sheet (Base Phase Structure: a Mixed Structure ofTempered Martensite and Ferrite)

[0520] In this Example, various manufacturing conditions shown in Tables18 and 19 were adopted using No. 4 slabs for experiment shown in Tables1 and 15, respectively, (the thickness of each hot rolled steel sheetwas set at 2.0 mm).

[0521] Next, the structure of each of the steel sheets was checked inthe same manner as in Example 1. The results obtained are also shown inTables 18 and 19. The steels used in this Example are different in onlythe amount of C (C of No. 4 in Table 1 is 0.20% and that of No. 4 inTable 15 is 0.48%) but are substantially the same in the contents ofother components, so that all of the structural constructions (types ofsecond phase) obtained were the same.

See Tables 18, 19

[0522] No. 1 to 25 in Table 18 were produced by the foregoing method(3). More specifically, No. 1 to 23 were subjected to hotrolling→continuous annealing. In No. 5 to 7 and No. 25 there wasconducted one-step cooling in the hot rolling process, while in theother runs there was conducted two-step cooling in the same process. No.24 and 25 were subjected to hot rolling→plating (further, alloying), ofwhich No. 24 is an example of having conducted two-step cooling in thehot rolling process and No.25 is an example of having conducted one-stepcooling in the same process.

[0523] No. 1, 3 to 4, 7, 9 to 11, 13 to 14, 16, 18 to 19, and 22 to 25are example of production carried out using conditions specified in thepresent invention, in which desired structures were obtained.

[0524] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 24 in Table 18 was used andheat-treated under the conditions shown in the same table with theproviso that pre-plating was applied thereto, to afford an alloyed, hotdip galvanized steel sheet. The details of the pre-plating are asdescribed in Example 3.

[0525] Thus, the alloyed, hot dip galvanized steel sheet having beensubjected to Fe pre-plating proved to have a good structure equal tothat obtained without going through pre-plating, and also proved to haveextremely superior plating characteristics (not shown in the table) suchas superior sliding property and powdering resistance of the platedsurface without the lack of plating.

[0526] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0527] No. 2 is an example of a high winding temperature (CT) in hotrolling, in which ferrite and tempered bainite were produced.

[0528] No. 5 is an example of a high CT, in which the same structure asin a conventional TRIP steel (TRIP steel with a base phase beingpolygonal ferrite) was obtained, but a desired structure was notobtained.

[0529] No. 6 is an example of a low average cooling rate (CR1) in hotrolling, in which, due to the absence of tempered martensite in theas-hot-rolled structure, a desired structure was not obtained and aconventional TRIP steel structure was produced.

[0530] No. 8 is an example of a high two phase region temperature (T3)in continuous annealing, in which a desired texture was not obtained anda conventional TRIP steel structure was produced.

[0531] No. 12 is an example of a low T3, in which desired γ_(R) was notobtained.

[0532] No. 15 is an example of a short holding time (t3) at a two phaseregion temperature in continuous annealing, in which tempering wasinsufficient and desired tempered martensite was not obtained.

[0533] No. 17 is an example of a low average cooling rate (CR) incontinuous annealing, in which pearlite was produced.

[0534] No. 20 and 21 are examples of a low austempering temperature (T4)(i.e., austempering was not performed), in which desired structure wasnot obtained and martensite was produced.

[0535] Next, No. 26 to 30 in Table 19 are example of having performedcold rolling in the foregoing method (3). More specifically, No. 26 to28 are example of having gone through hot rolling→coldrolling→continuous annealing and No. 29 to 30 are examples of havinggone through hot rolling→cold rolling→plating (further, alloying), ofwhich No. 28 and 30 are examples in which one-step cooling was performedin the hot rolling process, and the other examples adopted two-stepcooling.

[0536] No. 26 and 28 to 30 are examples using conditions specified inthe present invention, in which desired structures were obtained.

[0537] On the other hand, No. 27 is an example of a high cold rollingrate, in which pre-structure was destroyed by cold rolling and a desiredtempered martensite was not obtained.

[0538] Lastly, No. 31 to 57 in Table 19 followed the foregoing method(4). More specifically, No. 31 to 56 have gone through hot rolling→coldrolling→first continuous annealing→second continuous annealing, whileNo. 57 has undergone hot rolling→cold rolling→first continuousannealing→plating (further, alloying).

[0539] No. 31 to 34, 36, 37, 39, 41 to 43, 46 to 47, 49, 51 to 52, and55 to 57 adopted conditions specified in the present invention, in whichdesired structures were obtained.

[0540] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 57 in Table 19 was used and wassubjected to Fe pre-plating and alloying under the same conditions as inNo. 24 in Table 7 described previously. As a result, as is the case withthe omission of pre-plating, the alloyed, hot dip galvanized steel sheetthus Fe pre-plated afforded a satisfactory structure and was extremelysuperior in plating characteristic (not shown in the table) such asexcellent sliding property and powdering resistance of the platedsurface without the lack of plating.

[0541] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0542] No. 35 is an example of a low Ti, in which a desired γ_(R) wasnot obtained.

[0543] No. 38 is an example of a low average cooling rate (CR) in thefirst continuous annealing process, in which polygonal ferrite andpearlite were produced.

[0544] No. 40 is an example of a high two phase region temperature (T3)in the second continuous annealing process, in which a conventional TRIPsteel structure was obtained.

[0545] No. 44 is an example of a low T3, in which a desired γ_(R) wasnot obtained.

[0546] No. 45 is an example of a long holding time (t3) in two phaseregion in the second continuous annealing process, in which ferritestructure was produced as a base phase and tempered martensite vanished.

[0547] No. 48 is an example of a short t3, in which tempering wasinsufficient and desired tempered martensite was not obtained.

[0548] No. 50 is an example of a low average cooling rate (CR) in thesecond continuous annealing process, in which pearlite was produced.

[0549] No. 53 and 54 are examples low in austempering temperature (T4)(i.e., austempering is not performed), in which martensite was producedand a desired structure was not obtained.

Example 10 A Study (Part 1) of Components' Compositions in the FirstHigh Strength Steel Plate (Base Phase Structure: a Mixed Structure ofTempered Bainite and Ferrite)

[0550] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tolow C steels having a C content of 0.25% or less [steels high instrength (TS)×stretch flange formability (λ) and taking weldability intoaccount]. More specifically, steel samples comprising components'compositions shown in Table 1 (unit in the table is mass %) werevacuum-melted into slabs for experiment and thereafter the procedure ofExample 1 was repeated in accordance with the foregoing method (3) (hotrolling→continuous annealing) to afford hot rolled steel sheets having athickness of 2.0 mm.

[0551] Then in the same manner as in Example 1 the steel sheets thusobtained were measured for tensile strength (TS), elongation [totalelongation (El)], yield strength (YP), and stretch flange formability(hole expanding property: λ), and also there were measured an areafraction of structure in each of the steel sheets, a space factor ofγ_(R), and the concentration of C in γ_(R).

[0552] The results obtained are shown in Table 20.

See Table 20

[0553] The following can be seen from these results (all of thefollowing No. mean Run No. in Table 20).

[0554] First, all of No. 3 to 6, 8 to 18, and 20, which satisfy theconditions specified in the present invention, afforded steel sheets ofgood characteristics.

[0555] For reference, an optical microphotograph (magnification: 1000×)of a steel sheet (No. 3) according to the present invention is shown inFIG. 19. From this photograph it is seen that the steel sheet accordingto the present invention has tempered bainite of a lath structure andferrite.

[0556] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0557] First, No. 1 is an example of small amount C, in which it wasimpossible to attain a desired El.

[0558] No. 2 is an example of a CγR quantity of less than 0.8% , inwhich it was impossible to attain a desired El.

[0559] No. 7 is an example of a small amount of Mn and a small totalamount of (Si+Al), in which a desired γ_(R) was not obtainedand-therefore El was low.

[0560] No. 19 is an example of having adopted a low cooling rate and aconsequent large proportion of pearlite structure, in which apredetermined tempered bainite was not obtained and both El and λ weredeteriorated.

Example 11 A Study (Part 2) of Components' Compositions in the FirstHigh Strength Steel Sheet (Base Phase Structure: a Mixed Structure ofTempered Bainite and Ferrite)

[0561] In this Example a check was made about the influence of varyingcomponents' compositions on mechanical properties mainly with respect tohigh C steels having a C content of 0.25 to 0.6% [steels high instrength (TS)×stretch flange formability (λ) and also high inTS×elongation (El)]. More specifically, steel samples comprisingcomponents' compositions shown in Table 15 (unit in the table is mass %)were vacuum-melted, then hot rolled steel sheets were produced in thesame way as in Example 1 and were evaluated for characteristics.

[0562] The results obtained are shown in Table 21.

See Table 21

[0563] The following can be seen from these results (all of thefollowing No. mean Run No. in Table 21).

[0564] First, all of No. 3 to 6, 8 to 18, and 20, which satisfy theconditions specified in the present invention, afforded steel sheets ofgood characteristics.

[0565] For reference, an optical microphotograph (magnification: 1000×)of a steel sheet (No. 3) according to the present invention is shown inFIG. 20. From this photograph it is seen that the steel sheet accordingto the present invention has tempered bainite of a lath structure andferrite.

[0566] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0567] First, No. 1 is smaller in the amount of C, which is 0.15% , thanthe other examples (C: 0.4% or more) and is therefore low in El.

[0568] No. 2 is also as low as 0.20% in the amount of C and has a Cγ_(R)content of less than 0.8% , in which El is low.

[0569] No. 7 is an example of a small amount of Mn and a small totalamount of (Si+Al), in which a desired γ_(R) was not obtained and henceEl was low.

[0570] No. 19 is an example of having adopted a somewhat low coolingrate and a consequent large proportion of pearlite structure, in which apredetermined tempered bainite was not obtained and both El and λ weredeteriorated.

[0571] For reference, two types of steel sheets (a conventional TRIPsteel sheet using polygonal ferrite as a base phase and a conventionaltwo phase steel sheet of ferrite and bainite) were produced by usingsteel samples No. 2 and No. 3 shown in Table 1 and by suitably adjustingheat treatment conditions and were then evaluated for variouscharacteristics, the results of which are set out in table 22.

See Table 22

[0572] Reference to Table 22 shows that the conventional TRIP steelsheet using No. 3 in Table 1 is high in El but low in λ and that theconventional ferrite-bainite two phase steel sheet using No. 2 in Table1 is low in El.

Example 12 A Study of Manufacturing Conditions for the First HighStrength Steel Sheet (Base Phase Structure: a Mixed Structure ofTempered Bainite and Ferrite)

[0573] In this Example, various manufacturing conditions shown in Tables23 and 24 were adopted using No.4 slabs for experiment shown in Tables 1and 15, respectively, (the thickness of each hot rolled steel sheet isassumed to be 2.0 mm).

[0574] Next, the structure of each of the steel sheets was checked inthe same way as in Example 1. The results obtained are also set out inTables 23 and 24. The steels used in this Example are different in onlythe amount of C (C of No. 3 in Table 1 is 0.20% and that of No. 4 inTable 15 is 0.48%) but are substantially the same in the contents ofother components, so that all of the structures obtained were the same.

See Tables 23, 24

[0575] First, No. 1 to 25 in Table 23 were produced by the foregoingmethod (3). More specifically, No. 1 to 23 were subjected to hotrolling→continuous annealing, of which No. 5 to 7 and No. 25 adoptedone-step cooling in the hot rolling process and the others adoptedtwo-step cooling. Further, No. 24 and 25 are examples of having beensubjected to hot rolling→plating (further, alloying), of which No. 24 isan example of having adopted two-step cooling in the hot rolling processand No. 25 is an example of having adopted one-step cooling.

[0576] No. 1 to 3, 7, 9 to 11, 13, 14, 16, 18, 19, and 22 to 25 areexamples of production carried out using conditions specified in thepresent invention, in which desired structures were obtained.

[0577] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 24 in Table 23 was used andheat-treated under the conditions set out in Table 23 with the provisothat pre-plating was applied thereto, to afford an alloyed, hot dipgalvanized steel sheet. The details of the pre-plating are as describedin Example 3.

[0578] The alloyed, hot dip galvanized steel sheet thus Fe pre-platedafforded a satisfactory structure and was extremely superior in platingcharacteristics (not shown in the table) such as excellent slidingproperty and powdering resistance of the plated surface without the lackof plating.

[0579] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0580] No. 4 is an example of a low winding temperature (CT) in hotrolling, in which ferrite and tempered martensite were produced.

[0581] No. 5 is an example of a high CT, in which there was obtained thesame structure as that of a conventional TRIP steel (with a base phasebeing polygonal ferrite) and a desired structure was not obtained.

[0582] No. 6 is an example of a low average cooling rate (CR) in hotrolling, in which a desired structure was not obtained because ofabsence of tempered bainite in the as-hot-rolled structure, and aconventional TRIP steel structure was produced.

[0583] No.8 is an example of a high two phase region temperature (T3) incontinuous annealing, in which a desired structure was not obtained, buta conventional TRIP steel structure was produced.

[0584] No. 12 is an example of a low T3, in which γ_(R) structure wasnot obtained.

[0585] No. 15 is an example of a short holding time (t3) at a two phaseregion temperature in continuous annealing, in which tempering wasinsufficient and desired tempered bainite was not obtained.

[0586] No. 17 is an example of a low average cooling rate (CR) incontinuous annealing, in which pearlite was produced.

[0587] No. 20 and 21 are examples low in austempering temperature (T4)(i.e., austempering is not performed), in which a desired structure wasnot obtained, but martensite was produced.

[0588] Next, No. 26 to 30 in Table 23 are examples in which cold rollingwas performed in the foregoing method (3). More specifically, No. 26 to28 are examples which were subjected to hot rolling→coldrolling→continuous annealing and No. 29 and 30 are examples which weresubjected to hot rolling→cold rolling→plating (further, alloying). InNo. 28 and 30 there was adopted one-step cooling in the hot rollingprocess, while in the other examples there was adopted two-step cooling.

[0589] In No. 26 and 28 to 30 there were adopted conditions specified inthe present invention to afford desired structures.

[0590] On the other hand, No. 27 is an example of a high cold rollingrate, in which a desired tempered bainite was not obtained.

[0591] Lastly, No. 31 to 57 in Table 24 followed the foregoing method(4). More specifically, No. 31 to 56 have gone through hot rolling→coldrolling→first continuous annealing→second continuous annealing, whileNo. 57 has gone through hot rolling→cold rolling→first continuousannealing→plating (further, alloying).

[0592] No. 32 to 34, 36, 37, 39, 41 to 43, 46 to 47, 49, 51 to 52, and55 to 57 are examples of production carried out under conditionsspecified in the present invention, in which desired structures wereobtained.

[0593] For making sure the effect of improvement in platingcharacteristics by Fe pre-plating, No. 57 in Table 24 was subjected toFe pre-plating and alloying under the same conditions as No. 24. Thethus Fe pre-plated, alloyed, hot dip galvanized steel sheet proved tohave a good structure equal to that obtained without going throughpre-plating, and also proved to have extremely superior platingcharacteristics (not shown in the table) such as superior slidingproperty and powdering resistance of the plated surface without the lackof plating.

[0594] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0595] No. 31 is an example of a high γ region temperature (T1) in thecontinuous annealing process, in which not tempered bainite but ferriteand tempered martensite were produced.

[0596] No. 35 is an example of a low T1, in which a desired γ_(R)structure was not obtained.

[0597] No. 38 is an example of a low average cooling rate (CR) in thefirst continuous annealing process, in which polygonal ferrite andpearlite were produced.

[0598] No. 40 is an example of a high two phase region temperature (T3)in the second continuous annealing process, in which a conventional TRIPsteel structure was obtained.

[0599] No. 44 is an example of a low T3, in which a desired γ_(R) wasnot obtained.

[0600] No. 45 is an example of a long holding time (t3) in a two phasetemperature region in the second continuous annealing process, in whichferrite structure was obtained as a base phase structure, and temperedbainite vanished.

[0601] No. 48 is an example of a short t3, in which tempering wasinsufficient and a desired tempered bainite was not obtained.

[0602] No. 50 is an example of a low average cooling rate (CR) in thesecond continuous annealing process, in which pearlite was produced.

[0603] No. 53 and 54 are examples low in austempering temperature (T4)(i.e., austempering is not performed), in which martensite was producedand a desired structure was not obtained.

[0604] From the results of the above Examples 1 to 12 it is seen that ina high strength and ultra-high strength region of the order of about 500to 1400 MPa the first high strength steel sheet according to the presentinvention exhibits both excellent stretch flange formability andexcellent total elongation.

Example 13 A Study of Components' Compositions in the Second HighStrength Steel Sheet

[0605] In this Example, steel samples comprising components'compositions described in Table 25 (unit in the table is mass %) werevacuum-melted into slabs for experiment, from which there were obtainedcold rolled steel sheets having a thickness of 1.0 mm in accordance withthe method described in Table 26 [the foregoing method (8) (hotrolling→cold rolling→first continuous annealing→tempering→secondcontinuous annealing)].

[0606] Then, in the same way as in Example 1 the steel sheets thusobtained were measured for tensile strength (TS), elongation [totalelongation (El)], yield strength (YP), and stretch flange formability(hole expanding property: λ). As to the fatigue characteristic [fatigueendurance ratio (fatigue strength/yield strength), a fatigue limit wasdetermined by an endurance limit test under reverse stress and repeatedbending, then a fatigue endurance ratio [fatigue strength σ_(w)(MPa)/yield strength YP (MPa)] was calculated using the fatigue limit toevaluate the fatigue characteristic.

[0607] Further, in accordance with the foregoing method, a space factorof the structure in each of the steel sheets was measured and an arearatio [(S1/S)×100] of a coarse second phase structure was calculated.The amount of γ_(R) and the concentration of C in γ_(R) were measured byX-ray diffractometry after grinding to a quarter depth of each steelsheet and after subsequent chemical polishing (ISIJ Int. Vol.33 (1933),No.7, P.776).

[0608] The results obtained are shown in Table 27.

[0609] In Table 27 and in the column of the ratio of a coarse secondphase structure [(S1/S)×100], “-” means that γ_(R) which constitutes thesecond phase structure is not present or the amount thereof is verysmall, with no martensite produced, and that therefore it was impossibleto measure S1.

See Tables 25, 26, 27

[0610] The following can be seen from these results. All of thefollowing No. mean Run No. in Table 27. The examples described in Table27, which satisfy the condition of [S1/S]×100≦20], are for showingdifferences in other conditions (components and whether tempering isperformed or not).

[0611] First, all of No. 3 to 5and7 to 14 satisfy the conditionsspecified in the present invention and are therefore 10% or more higherin stretch flange formability (λ) and fatigue characteristic (σ_(w)/YP)than in case of steel of the same components having been heat-treatedwithout going through a predetermined tempering treatment (note: evenwhen a tempering treatment is not performed, if a predetermined heattreatment capable of being regarded as equal to the tempering treatmentis applied, it is regarded that tempering has been conducted).

[0612] On the other hand, No. 1 is an example of a low content of C, inwhich it was impossible to ensure a desired El, provided its fatiguecharacteristic is satisfactory because the second phase structure(γ_(R)/martensite) defined in the present invention was not produced.

[0613] No. 2 is an example of omission of a predetermined temperingtreatment, in which it was impossible to ensure a desired El and thefatigue characteristic was deteriorated.

[0614] No. 6 is an example of a small total amount of (Si+Al), in whicha desired El was not obtained.

[0615] No. 15 is an example of a low cooling rate and consequentproduction of a large amount of pearlite structure, in which El and λwere deteriorated.

[0616] For reference, evaluation results of various characteristics ofconventional steel sheets are shown in Table 28. In the same table, No.20 is a DP steel sheet of ferrite and martensite using No. 2 steelsample in Table 1, No. 21 is a conventional TRIP steel sheet usingNo. 2steel sample in Table 1 and with polygonal ferrite as a base phase, andNo. 22 is a conventional two phase steel sheet of ferrite and bainiteusing No. 2 steel sample in Table 1.

See Table 28

[0617] From Table 28 it is seen that No. 20 (conventional DP steelsheet) is inferior in all of elongation, stretch flange formability, andfatigue characteristic.

[0618] No. 21 (conventional TRIP steel sheet) contains a largeproportion of a coarse second phase structure and is inferior in bothstretch flange formability and fatigue characteristic.

[0619] No. 22 (conventional two phase steel sheet) is superior infatigue characteristic but inferior in elongation because of absence ofthe second phase structure defined in the present invention.

Example 14 A Study (Part 1) of Manufacturing Conditions for the SecondHigh Strength Steel Sheet

[0620] In this Example a study was made about the foregoingmanufacturing method (5) or (7), i.e., the method comprising hotrolling→tempering→continuous annealing. More specifically, No. 3 steelsample in Table 25 was vacuum-melted into a slab for experiment, fromwhich there were produced hot rolled steel sheets 2.0 mm thick under theconditions set out in Table 29. The steel sheets were then checked forstructure and characteristics in the same manner as in Example 13. InTable 29, No. 1, 2, and 5 are examples in which one-step cooling wasconducted in the hot rolling process, while in the other examples therewas conducted two-step cooling (after cooling to 700° C. at an averagecooling rate of 40° C./s, air-cooling was performed in this temperaturerange for 10 seconds, followed by cooling to 200° C. or 450° C. at anaverage cooling rate of 40° C./s) The results obtained are shown inTable 30.

See Tables 29, 30

[0621] In Table 30, No. 2 is an example according to the presentinvention in which a desired base phase structure of tempered martensitewas obtained through predetermined steps of hotrolling→tempering→continuous annealing, No. 4 is an example according tothe present invention in which a desired mixed base phase structure of(tempered martensite+ferrite) was obtained through predetermined hotrolling→tempering→continuous annealing, No. 5 is an example according tothe present invention in which a desired base phase structure oftempered bainite was obtained through predetermined steps of hot rolling(tempering can be omitted because a winding process is performed at a CTof 450° C. for 1 hour)→continuous annealing, and No. 6 is an exampleaccording to the present invention in which a desired mixed base phasestructure of (tempered bainite+martensite) was obtained throughpredetermined steps of hot rolling (tempering can be omitted because awinding process is performed at a CT of 450° C. for 1 hour)→continuousannealing. All of these examples, due to formation of fine second phasestructures, are 10% ormore higher in stretch flange formability (λ) andfatigue characteristic (σ_(w)/YP) than in case of steel of the samecomponents having been heat-treated without going through apredetermined tempering treatment (note: even when a tempering treatmentis not performed, if a predetermined heat treatment capable of beingregarded as equal to the tempering treatment is applied, it is regardedthat tempering has been conducted).

[0622] On the other hand, No. 1 and 3 in Table 30 are examples ofproduction carried out without going through tempering, which are low infatigue characteristic or in both fatigue characteristic and stretchflange formability due to a large proportion of a coarse second phasestructure.

Example 15 A Study (Part 2) of Manufacturing Conditions for the SecondHigh Strength Steel Sheet

[0623] In this Example a study was made about the foregoing method (6)or (8), i.e., hot rolling→cold rolling→first continuousannealing→tempering→second continuous annealing. More specifically,various steels shown in Tables 31 and 33 (the steel Nos. described inTables 31 and 33 mean the steel Nos. in Table 25) were vacuum-meltedinto slabs for experiment. Using these slabs, cold rolled steel sheets1.0 mm thick were produced under the heat treatment conditions shown inTables 31 and 33 and were then checked for structure and characteristicsin the same manner as in Example 13. No. 1 to 34 in Table 31 weresubjected to hot rolling→cold rolling→first continuousannealing→(tempering)→second continuous annealing, while No. 1 to 6 weresubjected to hot rolling→cold rolling→first continuousannealing→(tempering)→plating (further, alloying). The results obtainedin Table 31 is shown in Table 32 and the results obtained in Table 33are shown in Table 34.

[0624] In Table 32 and in the column of the proportion of a coarsesecond phase structure [(S1/S)×100], “-” means that γ_(R) whichconstitutes the second phase structure is not present or the amountthereof is very small, with no martensite produced, and that thereforeit was impossible to measure S1.

See Tables 31, 32, 33, 34

[0625] First, No. 4, 7 to 9, 13, 16, 20, 22, 24, 26, 28, 30, 32, and 34in Table 32 are examples of production carried out under conditionsdefined in the present invention, which are 10% or more higher instretch flange formability (λ) and fatigue characteristic (σ_(w)/YP)than in case of steel of the same components having been heat-treatedwithout going through a predetermined tempering treatment (note: evenwhen a tempering treatment is not performed, if a predetermined heattreatment capable of being regarded as equal to the tempering treatmentis applied, it is regarded that tempering has been conducted).

[0626] In contrast therewith, the following examples lacking in any ofthe conditions specified in the present invention have the followingdisadvantages.

[0627] No. 1.and 2 in Table 32 are examples of production using steel 1(low C steel) shown in Table 1, in which a predetermined base phasestructure was obtained, but due to a small amount of C there was notobtained a desired γ_(R) and TS×El were low.

[0628] No. 3, 5, 11 to 12, 14 to 15, 19, 21, 23, 25, 27, 29, 31, and 33,in Table 32, as well as No. 1, 3 to 4, and 6 in Table 34, are allexamples of production performed without going through tempering, inwhich fatigue characteristic or both fatigue characteristic and stretchflange formability were deteriorated due to a large proportion of acoarse second phase structure.

[0629] No. 6 in Table 32 is an example of a low tempering temperature,in which stretch flange formability and fatigue characteristic weredeteriorated.

[0630] No. 10 in Table 32 is an example of a long-time treatmentconducted at a high tempering temperature, in which stretch flangeformability and fatigue characteristic were deteriorated.

[0631] No.17 and 18 in Table 32 are examples of production using steel 5[steel having a small total amount of (Si+Al)] in Table 25, in which adesired γ_(R) was not produced and elongation was deteriorated.

[0632] For reference, a photograph (magnification: 4000×) taken throughan SEM (scanning electron microscope) of a steel sheet according to thepresent invention (No. 13 in Table 32) and that of a comparative steelsheet (No. 12 in Table 32) are shown in FIGS. 21 and 22, respectively.From these photographs it is seen that the steel sheet according to thepresent invention afforded a desired structure [a base phase structure(tempered martensite) of a lath form and a fine second phase structure]because it was treated under conditions specified in the presentinvention, but that the comparative steel sheet of FIG. 22 cannot afforda desired structure (a coarse second phase structure was formed) due toomission of a predetermined tempering treatment.

[0633] From the above results obtained in Examples 13 to 15 it is seenthat the second high strength steel sheet according to the presentinvention is superior in the balance of stretch flange formability,total elongation, and fatigue characteristic in a high strength andultra-high strength region of the order of about 500 to 1400 MPa.

Example 16 A Study of Components' Compositions and Heating Temperature(SRT) Before Hot Rolling in the Third High Strength Steel Sheet

[0634] In this Example, steel samples comprising components'compositions shown in Table 35 (unit in the table is mass %) werevacuum-melted into slabs for experiment. Thereafter, using the slabs,cold rolled steel sheets having a thickness of 1.0 mm were produced inaccordance with the method described in Table 36 [the foregoing method(12) (hot rolling→cold rolling→first continuous annealing→secondcontinuous annealing)].

[0635] Then, in the same manner as in Example 1, the steel sheets thusproduced were measured for tensile strength (TS), elongation [totalelongation (El)], and stretch flange formability (hole expandingproperty: λ).

[0636] The results obtained are shown in Table 37.

See Tables 35, 36, 37

[0637] The following can be seen from these results. All of thefollowing No. mean Run No. in Table 37.

[0638] First, it is seen that all of No. 2 to 4, 6 to 13, and 15 to 20,which satisfy the conditions specified in the present invention, aresuperior in strength (TS), elongation (El), and stretch flangeformability, and that they have a very excellent bake hardening propertybecause they satisfy the conditions specified in the present inventionalso with respect to BH (2%) and BH (10%).

[0639] On the other hand, No. 1 is an example of a small amount of C, inwhich it was impossible to obtain desired BH characteristics.

[0640] No. 5 is an example of a small total amount of (Si+Al), in whicha desired El is hot obtained and BH characteristics are alsodeteriorated markedly.

[0641] No.14 is an example of a low cooling rate anda consequentformation of a large amount of pearlite structure as a second phasestructure, in which El and λ are low and BH characteristics are alsoinferior.

[0642] Next, for the purpose of checking the influence of the heatingtemperature (SRT) before hot rolling on BH characteristics [especiallyBH (10%)], steel samples shown in Table 35 were used and cold rolledsteel sheets having a thickness of 1.0 mm were produced in accordancewith the method described in Table 38. Thesteel sheets thus obtainedwere then checked for predetermined mechanical characteristics in thesame manner as above, the results of which are given in Table 39.

See Tables 38, 39

[0643] From these results it is seen that if steel sheets are producedat an SRT deviated from the range (950 to 1100° C.) defined in thepresent invention, BH (2%) is substantially the same or a little lower,but BH (10%) is markedly deteriorated, not affording a steel sheetmeeting the conditions defined in the present invention, in comparisonwith the case of Table 37 in which SRT was controlled to the rangedefined in the present invention.

[0644] For reference, various characteristics of conventional steelsheets were evaluated, the results of which are shown in Table 40. Inthe same table, No. 1 is a DP steel sheet of ferrite and martensiteproduced using No. 2 steel sample in Table 35, No. 2 is a conventionalsteel sheet using No. 3 steel sample in Table 35 and with polygonalferrite as a base phase, and No. 3 is a conventional two phase steelsheet of ferrite and bainite produced using No. 2 steel sample in Table35.

See Table 40

[0645] A look at Table 40 shows that No.1 (conventional DP steel sheet)is low in all of elongation, stretch flange formability, and BHcharacteristics, that No. 2 (conventional TRIP steel sheet) is low inboth stretch flange formability and BH characteristics, and that No. 3(conventional two phase steel sheet) is low in both elongation and BHcharacteristics.

Example 17 A Study (Part 1) of Manufacturing Conditions in the ThirdHigh Strength Steel Sheet

[0646] In this Example a study was made about the foregoingmanufacturing method (9) or (11), i.e., hot rolling→continuousannealing. More specifically, steel No. 3 in Table 35 was vacuum-meltedinto a slab for experiment. Thereafter, using the slab, 2.0 mm thick hotrolled steel sheets were produced under the conditions shown in Table 41and were checked for structures and characteristics in the same manneras in Example 16. In Table 41, No. 1, 3, 5, and 7 are examples in whichone-step cooling was adopted in the hot rolling process, while No. 2, 4,6, and 8 are examples in which two-step cooling (cooling to 700° C. atan average cooling rate of 40° C./s is followed by air cooling in thistemperature range for 10 seconds and subsequent cooling to 200° C. or450° C. at an average cooling rate of 40° C./s) was adopted.

[0647] The results obtained are shown in Table 42.

See Tables 41, 42

[0648] In Table 42, No. 1 is an example of the present invention inwhich a desired base phase structure of tempered martensite was obtainedthrough predetermined steps of hot rolling→continuous annealing, No. 2is an example of the present invention in which a desired mixed basephase structure of (tempered martensite+ferrite) was obtained throughpredetermined steps of hot rolling→continuous annealing, No. 3 is anexample of the present invention in which a desired base phase structureof tempered bainite was obtained through predetermined steps of hotrolling (winding at a CT of 450° C. for 1 hour)→continuous annealing,and No. 4 is an example of the present invention in which a desiredmixed base phase structure of (tempered bainite+ferrite) was obtainedthrough predetermined steps of hot rolling (winding at a CT of 450° C.for 1 hour). All of them are superior in stretch flange formability;besides, in all of them, a desired fine second phase structure isdispersed uniformly in pre-austenite grain boundaries and block andpacket boundaries, and thus their BH characteristics also meet theconditions defined in the present invention.

[0649] On the other hand, No. 5 to 8 in Table 42 are examples ofproduction carried out at aheating temperature (SRT) (before hotrolling) exceeding the range defined in the present invention, in whicha desired fine second phase structure is not obtained and therefore BH(10%) does not satisfy the condition defined in the present inventionalthough BH (2%) is satisfactory.

Example 18 A Study (Part 2) of Manufacturing Conditions in the ThirdHigh Strength Steel Sheet

[0650] In this Example a study was made about the foregoingmanufacturing method (10) or (12), i.e., hot rolling→cold rolling→firstcontinuous annealing→second continuous annealing or plating. Morespecifically, various steels described in Tables 43 and 45 (all of steelNos. described in these tables mean the steel Nos. described in Table35) were vacuum-melted into slabs for experiment. Thereafter, using theslabs, cold rolled steel sheets having a thickness of 1.0 mm wereproduced under the heat treatment conditions shown in Tables 43 and 45and were then checked for structures and characteristics in the samemanner as in Example 1. No. 1 to 16 in Table 43 are examples of havingstudied hot rolling→cold rolling→first continuous annealing→secondcontinuous annealing, while No. 1 to 4 in Table 45 are examples ofhaving studied hot rolling→cold rolling→first continuousannealing→plating (further, alloying). The results obtained in Tables 43and 45 are shown in Tables 44 and 46, respectively.

See Tables 43, 44, 45

[0651] No. 2 to 7 and 9 to 20 in Table 44 and No. 1 to 4 in Table 46 areexamples of production carried out under the conditions defined in thepresent invention, which are superior not only in tensile strength (TS),elongation (EL), and stretch flange formability (λ), but also in both BH(2%) and BH (10%).

[0652] On the other hand, No. 1 in Table 44 is an example of productionusing steel 1 (low C steel) shown in Table 35, in which desired BHcharacteristics were not obtained because of a small amount of Calthough a predetermined base phase structure was produced.

[0653] No. 8 in Table 44 is an example of production using steel 5{steel having a small total amount of (Si+Al)} shown in Table 35, inwhich desired BH characteristics were not obtained.

[0654] Next, in this embodiment, for the purpose of checking theinfluence of the heat treatment temperature (SRT) before hot rolling onBH characteristics [especially BH (10%)], as shown in Table 47, steelsheets were produced under the conditions given in Table 43 except thatSRT was raised from 1050° C. to 1150° C. (exceeding the range defined inthe present invention), and were then checked for various mechanicalcharacteristics in the same manner as in Example 1, the results of whichare set out in Table 48.

See Tables 47, 48

[0655] From a comparison in mechanical properties between Tables 44 and48 it is seen that if SRT is set higher than in the present invention asin Table 48, BH (2%) is approximately the same, but BH (10%) isdeteriorated markedly beyond the point of satisfaction for the conditiondefined in the present invention, and such characteristics as TS, El,and λ are also deteriorated to some extent.

[0656] Likewise, for the purpose of checking the influence of SRT, asshown in Table 49, steel sheets were produced under the conditions shownin Table 45 except that SRT was raised from 1050° C. to 1150° C.(exceeding the range defined in the present invention) and were measuredfor various mechanical properties in the same way as in Example 1, theresults of which are shown in Table 50.

See Tables 49, 50

[0657] A comparison in mechanical properties between Tables 45 and 50shows that if SRT is set higher than in the present invention, BH (2%)is approximately the same, but BH (10%) is deteriorated markedly beyondthe point of satisfaction for the condition defined in the presentinvention, and such characteristics as TS, El, and λ are also somewhatdeteriorated.

[0658] Thus, from the results obtained in the above Examples 16 to 18 itis seen that the third high strength steel according to the presentinvention is superior in.the balance of stretch flange formability,total elongation, and bake hardening property in a high-strength andultra-high strength region of the order of about 500 to 1400 MPa andthat above all, in a large strain region, it exhibits an excellent bakehardening property.

Industrial Applicability

[0659] According to the present invention it is possible to provide ahigh strength steel sheet superior in formability (stretch flangeformability and total elongation), a high strength steel sheet alsohaving an excellent fatigue characteristic, further, a high strengthsteel sheet further having a satisfactory bake hardening property, aswell as a methodwhich canproducethosesteelsheetsefficiently. Thus, thepresent invention is extremely useful. TABLE 1 No. C Si Mn P S Al Others1 0.03 1.5 1.5 0.02 0.005 0.03 2 0.09 1.5 1.5 0.01 0.005 0.03 3 0.15 1.51.5 0.03 0.006 0.03 4 0.20 1.5 1.5 0.03 0.004 0.03 5 0.15 0.5 1.5 0.030.004 1.0 6 0.15 0.3 0.3 0.02 0.004 0.03 7 0.15 1.5 1.5 0.01 0.005 0.03Mo: 0.2 8 0.15 1.5 1.5 0.02 0.006 0.03 Ni: 0.2 9 0.15 1.5 1.5 0.02 0.0060.03 Cu: 0.2 10 0.15 1.5 1.5 0.02 0.005 0.03 Cr: 0.2 11 0.15 1.5 1.50.01 0.006 0.03 Ti: 0.03 12 0.15 1.5 1.5 0.02 0.005 0.03 Nb: 0.03 130.15 1.5 1.5 0.03 0.006 0.03 V: 0.03 14 0.15 1.5 1.5 0.01 0.005 0.03 Ca:10 ppm

[0660] Steel TM B γ_(R) Others C_(γR) TS El λ YR No. No. (%) (%) (%) (%)(%) (Mpa) (%) (%) (%) 1 1 45 5 0 50(F) — 469 32 74 78 2 2 79 8 8  5(F)1.4 590 38 66 70 3 3 84 6 10 0 1.4 801 39 68 73 4 4 80 7 13 0 1.5 880 3969 73 5 5 85 5 10 0 1.4 730 38 78 78 6 6 83 5 1 0 1.5 808 20 75 96 7 789 3 8 0 1.4 800 37 66 74 8 8 88 3 9 0 1.5 810 36 63 70 9 9 89 4 7 0 1.5803 38 65 71 10 10 87 3 10 0 1.4 793 39 67 72 11 11 87 3 10 0 1.4 799 3969 73 12 12 87 4 9 0 1.4 810 38 62 75 13 13 87 3 10 0 1.4 802 39 66 7314 14 88 3 9 0 1.5 803 38 65 74 15 4 80 0 13 7(M) 1.3 881 39 63 65

[0661] TABLE 3 Steel M B γ_(R) F C_(γR) No. No. (%) (%) (%) (%) (%) TS(Mpa) El (%) λ (%) YR (%) 1 2 23 3 0 74 — 850 22 43 52 2 3 0 4 12 84 1.4788 37 41 67 3 2 0 83 0 17 — 830 15 59 93

[0662] TABLE 4 No. C Si Mn P S Al Others 1 0.15 1.5 1.5 0.02 0.005 0.032 0.20 1.5 1.5 0.03 0.005 0.03 3 0.41 1.5 1.5 0.02 0.005 0.03 4 0.48 1.51.5 0.02 0.006 0.03 5 0.57 1.5 1.5 0.01 0.004 0.03 6 0.50 0.5 1.5 0.030.004 1.0 7 0.42 0.3 0.3 0.01 0.004 0.03 8 0.43 1.5 1.5 0.02 0.005 0.03Mo: 0.2 9 0.42 1.5 1.5 0.01 0.006 0.03 Ni: 0.2 10 0.40 1.5 1.5 0.020.006 0.03 Cu: 0.2 11 0.41 1.5 1.5 0.03 0.005 0.03 Cr: 0.2 12 0.42 1.51.5 0.01 0.006 0.03 Ti: 0.03 13 0.41 1.5 1.5 0.02 0.005 0.03 Nb: 0.03 140.42 1.5 1.5 0.02 0.006 0.03 V: 0.03 15 0.41 1.5 1.5 0.02 0.005 0.03 Ca:10 ppm

[0663] Steel TM B γ_(R) Others C_(γR) TS El λ YR No. No. (%) (%) (%) (%)(%) (Mpa) (%) (%) (%) 1 1 84 6 10 0 1.2 801 39 48 73 2 2 82 7 11 0 1.3830 39 49 73 3 3 73 7 20 0 1.4 860 51 50 75 4 4 67 9 24 0 1.6 910 55 5174 5 5 67 5 28 0 1.5 940 49 49 73 6 6 65 10 25 0 1.6 850 45 48 75 7 7 7920 1 0 0.2 890 20 50 95 8 8 75 4 21 0 1.6 870 51 49 76 9 9 74 5 21 0 1.7870 50 53 74 10 10 75 5 20 0 1.8 860 52 51 75 11 11 75 5 20 0 1.7 860 4952 75 12 12 74 5 21 0 1.8 850 50 49 73 13 13 74 6 20 0 1.7 860 51 51 7414 14 75 5 20 0 1.8 858 50 50 73 15 15 73 7 20 0 1.7 850 49 49 74 16 334 5 20 41(P) 0.6 780 21 33 83 17 4 68 0 27  5(M) 1.5 910 54 50 65

[0664] TABLE 6 M B γ_(R) F C_(γR) TS El λ YR No. (%) (%) (%) (%) (%)(Mpa) (%) (%) (%) 3 0 4 12 84 1.4 788 37 41 67

[0665] TABLE 7 Continuous Cold annealing rolling Continuous or De- Hotrolling Cold annealing plating sired Details SRT FDT CR CT rolling T1 CRT2 T3 t3 CR T4 t4 Zn→ struc- of No. ° C. ° C. ° C./s ° C. rate % ° C. °C./s ° C. ° C. sec ° C./s ° C. sec GA ° C. ture structure Hot 1 1150 90050 200 — — — — 800 60 10 400 10 — ◯ rolling → 2 1150 750 50 200 — — — —800 60 10 400 10 — X F Continuous 3 1150 900 60 200 — — — — 800 60 10400 10 — ◯ annealing 4 1150 900 15 200 — — — — 800 60 10 400 10 — X F, P5 1150 900 50 400 — — — — 800 60 10 400 10 — X B 6 1150 900 50 RT — — —— 800 60 10 400 10 — ◯ 7 1150 900 50 550 — — — — 800 60 10 400 10 — XConventional TRIP 8 1150 900 50 200 — — — — 900 60 10 400 10 — X B 91150 900 50 200 — — — — 850 60 10 400 10 — ◯ 10 1150 900 50 200 — — — —750 60 10 400 10 — ◯ 11 1150 900 50 200 — — — — 700 60 10 400 10 — ◯ 121150 900 50 200 — — — — 650 60 10 400 10 — X γ-less 13 1150 900 50 200 —— — — 800 180  10 400 10 — ◯ 14 1150 900 50 200 — — — — 800 30 10 400 10— ◯ 15 1150 900 50 200 — — — — 800  5 10 400 10 — X Insufficienttempering 16 1150 900 50 200 — — — — 800 60  5 400 10 — ◯ 17 1150 900 50200 — — — — 800 30  1 400 10 — X P 18 1150 900 50 200 — — — — 800 60 10450 10 — ◯ 19 1150 900 50 200 — — — — 800 60 10 350 10 — ◯ 20 1150 90050 200 — — — — 800 60 10 200 10 — X Austempering not performed, Mobtained 21 1150 900 50 200 — — — — 800 60 10 RT — — X Austempering notperformed, M obtained 22 1150 900 50 200 — — — — 800 60 10 400 100  — ◯23 1150 900 50 200 — — — — 800 60 10 400  1 — ◯ Hot 24 1150 900 50 200 —— — — 800 60 10 400 10 600 ◯ rolling → Plating Hot 25 1150 900 50 200 10— — — 800 60 10 400 10 — ◯ rolling → 26 1150 900 50 200 40 — — — 800 6010 400 10 — X F (tempered Cold M not rolling → obtained) Continuousannealing Hot 27 1150 900 50 200 10 — — — 800 60 10 400 10 600 ◯ rolling→ Cold rolling → Plating

[0666] TABLE 8 Continuous Cold annealing rolling Continuous or De- Hotrolling Cold annealing plating sired Details SRT FDT CR CT rolling T1 CRT2 T3 t3 CR T4 t4 Zn→ struc- of No. ° C. ° C. ° C./s ° C. rate % ° C. °C./s ° C. ° C. sec ° C./s ° C. sec GA ° C. ture structure Hot 28 1150900 50 550 60 900 20 RT 800  60 10 400 10 — ◯ rolling → 29 1150 900 50550 60 800 20 RT 800  60 10 400 10 — X F Cold 30 1150 900 50 550 60 70020 RT 800  60 10 400 10 — X F rolling → 31 1150 900 50 550 60 900 50 RT800  60 10 400 10 — ◯ First 32 1150 900 50 550 60 900 10 RT 800  60 10400 10 — ◯ continuous 33 1150 900 50 550 60 900  5 RT 800  60 10 400 10— X F, P annealing → 34 1150 900 50 550 60 900 20 200 800  60 10 400 10— ◯ Second 35 1150 900 50 550 60 900 20 RT 900  60 10 400 10 — X Bcontinuous 36 1150 900 50 550 60 900 20 RT 850  60 10 400 10 — ◯annealing 37 1150 900 50 550 60 900 20 RT 750  60 10 400 10 — ◯ 38 1150900 50 550 60 900 20 RT 700  60 10 400 10 — ◯ 39 1150 900 50 550 60 90020 RT 650  60 10 400 10 — X γ-less 40 1150 900 50 550 60 900 20 RT 8001000 10 400 10 — X F 41 1150 900 50 550 60 900 20 RT 800  180 10 400 10— ◯ 42 1150 900 50 550 60 900 20 RT 800  30 10 400 10 — ◯ 43 1150 900 50550 60 900 20 RT 800   5 10 400 10 — X Insufficient tempering 44 1150900 50 550 60 900 20 RT 800  60  5 400 10 — ◯ 45 1150 900 50 550 60 90020 RT 800  30  1 400 10 — X P 46 1150 900 50 550 60 900 20 RT 800  60 10450 10 — ◯ 47 1150 900 50 550 60 900 20 RT 800  60 10 350 10 — ◯ 48 1150900 50 550 60 900 20 RT 800  60 10 200 10 — X Austempering notperformed, M obtained 49 1150 900 50 550 60 900 20 RT 800  60 10 RT — —X Austempering not performed, M obtained 50 1150 900 50 550 60 900 20 RT800  60 10 400 100  — ◯ 51 1150 900 50 550 60 900 20 RT 800  60 10 400 1 — ◯ Hot 52 1150 900 50 550 50 900 20 RT 800  60 10 400 10 600 ◯rolling → Cold rolling → First continuous annealing → Plating

[0667] TABLE 9 Run Steel TB B γ_(R) Others C_(γR) TS El λ YR No. No. (%)(%) (%) (%) (%) (Mpa) (%) (%) (%) 1 1 47 3 0 50 (F) — 458 38 68 83 2 282 5 8  5 (F) 1.2 572 39 60 77 3 3 86 4 10 0 1.1 790 39 55 76 4 4 81 613 0 1.3 875 40 55 77 5 5 87 4 9 0 1.3 780 41 60 73 6 6 95 4 1 0 0.4 79010 71 103 7 7 87 5 8 0 1.3 799 38 55 79 8 8 86 5 9 0 1.3 809 37 50 75 99 87 6 7 0 1.4 800 39 54 76 10 10 85 5 10 0 1.3 799 38 58 77 11 11 84 610 0 1.3 789 39 61 76 12 12 88 3 9 0 1.4 807 37 66 82 13 13 86 4 10 01.3 800 39 54 75 14 14 88 3 9 0 1.4 800 37 58 76 15 4 80 0 14  6(M) 1.3881 39 54 70

[0668] TABLE 10 Steel TB B γ_(R) Others C_(γR) TS El λ YR No. No. (%)(%) (%) (%) (%) (Mpa) (%) (%) (%) 1 1 86 4 10 0 1.2 790 39 55 76 2 2 816 13 0 1.3 875 40 55 77 3 3 76 4 20 0 1.5 895 51 60 76 4 4 71 6 23 0 1.6900 50 59 75 5 5 75 4 25 0 1.7 910 49 58 73 6 6 72 5 23 0 1.8 870 48 5972 7 7 84 15 1 0 0.5 880 12 58 72 8 8 81 5 21 0 1.7 890 53 61 74 9 9 816 20 0 1.8 900 51 62 76 10 10 83 5 20 0 1.6 905 50 60 77 11 11 81 6 20 01.7 895 49 59 71 12 12 86 3 21 0 1.6 890 53 57 70 13 13 82 5 20 0 1.7860 51 58 69 14 14 82 6 20 0 1.8 880 50 61 73 15 15 82 4 21 0 1.7 890 4860 70 16 3 32 12 20 45(P) 0.7 695 21 43 75 17 4 70 0 25  5(M) 1.6 900 5050 65

[0669] TABLE 11 M B γR F C_(γR) TS EI λ YR No. (%) (%) (%) (%) (%) (Mpa)(%) (%) (%) 3 0 4 12 84 1.4 788 37 41 67

[0670] TABLE 12 Hot rolling Cold rolling Continuous annealing SRT FDT CRCT Cold rolling T1 CR T2 No. ° C. ° C. ° C./s ° C. rate % ° C. ° C./s °C. Hot rolling → 1 1150 900 50 480 — — — — Continuous annealing 2 1150750 50 480 — — — — 3 1150 900 60 480 — — — — 4 1150 900 15 200 — — — — 51150 900 50 RT — — — — 6 1150 900 50 750 — — — — 7 1150 900 50 380 — — —— 8 1150 900 50 380 — — — — 9 1150 900 50 380 — — — — 10 1150 900 50 380— — — — 11 1150 900 50 380 — — — — 12 1150 900 50 380 — — — — 13 1150900 50 380 — — — — 14 1150 900 50 380 — — — — 15 1150 900 50 380 — — — —16 1150 900 50 380 — — — — 17 1150 900 50 380 — — — — 18 1150 900 50 380— — — — 19 1150 900 50 380 — — — — 20 1150 900 50 380 — — — — 21 1150900 50 380 — — — — 22 1150 900 50 380 — — — — Hot rolling → Plating 231150 900 50 380 — — — — Hot rolling → Cold rolling → 24 1150 900 50 38010 — — — Continuous annealing 25 1150 900 50 380 40 — — — Hot rolling →Cold 26 1150 900 50 380 10 — — — rolling → Plating Continuous annealingor plating T3 t3 CR T4 t4 Zn→GA Desired ° C. sec ° C/s ° C. sec ° C.structure Details of structure Hot 800 60 10 400 10 — ◯ rolling → 800 6010 400 10 — X F Continuous 800 60 10 400 10 — ◯ annealing 800 60 10 40010 — X F, P 800 60 10 400 10 — X Tempered martensite 800 60 10 400 10 —X Conventional TRIP 900 60 10 400 10 — X B 850 60 10 400 10 — ◯ 750 6010 400 10 — ◯ 700 60 10 400 10 — ◯ 650 60 10 400 10 — X γ-less 800 18010 400 10 — ◯ 800 30 10 400 10 — ◯ 800  5 10 400 10 — X Insufficienttempering 800 60  5 400 10 — ◯ 800 30  3 400 10 — X P 800 60 10 450 10 —◯ 800 60 10 350 10 — ◯ 800 60 10 200 10 — X Austempering not performed800 60 10 RT — — X Austempering not performed 800 60 10 400 100 — ◯ 80060 10 400 1 — ◯ Hot 800 60 10 400 10 600 ◯ rolling → Plating Hot 800 6010 400 10 — ◯ rolling → 800 60 10 400 10 — X F (tempered M not obtained)Cold rolling → Continuous annealing Hot 800 60 10 400 10 600 ◯ rolling →Cold rolling → Plating

[0671] TABLE 13 Hot rolling Cold rolling Continuous annealing SRT FDT CRCT Cold rolling T1 CR T2 No. ° C. ° C. ° C./s ° C. rate % ° C. ° C./s °C. Hot rolling → 27 1150 850 40 550 60 900 20 480 rolling → First 281150 850 40 550 60 800 20 480 continuous annealing → 29 1150 850 40 55060 700 20 480 Second continuous 30 1150 850 40 550 60 900 50 480annealing 31 1150 850 40 550 60 900 10 480 32 1150 850 40 550 60 900  5480 33 1150 850 40 550 60 900 20 700 34 1150 850 40 550 60 900 20 480 351150 850 40 550 60 900 20 480 36 1150 850 40 550 60 900 20 480 37 1150850 40 550 60 900 20 480 38 1150 850 40 550 60 900 20 480 39 1150 850 40550 60 900 20 480 40 1150 850 40 550 60 900 20 480 41 1150 850 40 550 60900 20 480 42 1150 850 40 550 60 900 20 480 43 1150 850 40 550 60 900 20480 44 1150 850 40 550 60 900 20 480 45 1150 850 40 550 60 900 20 480 461150 850 40 550 60 900 20 480 47 1150 850 40 550 60 900 20 480 48 1150850 40 550 60 900 20 480 49 1150 850 40 550 60 900 20 480 50 1150 850 40550 60 900 20 480 Hot rolling → Cold 51 1150 850 40 550 50 900 20 480rolling → First continuous annealing → Plating Continuous annealing orplating T3 t3 CR T4 t4 Zn→GA Desired ° C. sec ° C/s ° C. sec ° C.structure Details of structure Hot 800 60 10 400 10 — ◯ rolling → 800 6010 400 10 — X F First 800 60 10 400 10 — X F continuous 800 60 10 400 10— ◯ annealing → 800 60 10 400 10 — ◯ Second 800 60 10 400 10 — X F, Pcontinuous 800 60 10 400 10 — ◯ annealing 900 60 10 400 10 — X B 850 6010 400 10 — ◯ 750 60 10 400 10 — ◯ 700 60 10 400 10 — ◯ 650 60 10 400 10— X γ-less 800 ###  10 400 10 — X 800 180  10 400 10 — ◯ 800 30 10 40010 — ◯ 800  5 10 400 10 — X lnsufficient tempering 800 60  5 400 10 — ◯800 30  3 400 10 — X P 800 60 10 450 10 — ◯ 800 60 10 350 10 — ◯ 800 6010 200 10 — X Austempering not performed 800 60 10 RT — — X Austemperingnot performed 800 60 10 400 100 — ◯ 800 60 10 400 1 — ◯ Hot 800 60 10400 10 600 ◯ rolling → Cold rolling → First continuous annealing →Plating

[0672] TABLE 14 Steel TM B γR F Others C_(γR) TS EI λ YR No. No. (%) (%)(%) (%) (%) (%) (Mpa) (%) (%) (%) 1 1 20 5 0 75 0 — 770 25 50 73 2 2 305 15 50 0 0.6 701 31 46 67 3 2 33 8 5 54 0 1.4 760 38 56 65 4 3 30 6 1054 0 1.5 820 39 58 68 5 4 33 7 13 47 0 1.5 810 39 59 68 6 5 42 6 12 40 01.4 790 38 60 67 7 6 39 5 1 55 0 1.2 800 14 65 91 8 7 33 3 8 56 0 1.4790 37 56 69 9 8 32 2 9 57 0 1.4 785 36 53 65 10 9 36 3 7 54 0 1.5 77038 55 66 11 10 32 4 9 55 0 1.4 780 39 57 67 12 11 26 5 8 61 0 1.5 805 3959 68 13 12 30 3 9 58 0 1.4 815 38 52 70 14 13 30 2 8 60 0 1.5 810 38 5964 15 14 26 6 9 59 0 1.5 790 39 58 68 16 2 72 5 8 15 0 1.3 750 40 66 7017 3 69 4 9 18 0 1.5 740 41 65 69 18 4 66 4 10 20 0 1.5 800 42 65 71 193 24 4 10 44 18(P) 1.3 770 29 43 78 20 4 33 0 13 49  5(M) 1.5 810 38 5761

[0673] TABLE 15 No. C Si Mn P S AI Others 1 0.15 1.5 1.5 0.02 0.005 0.032 0.20 1.5 1.5 0.03 0.005 0.03 3 0.41 1.5 1.5 0.02 0.006 0.03 4 0.48 1.51.5 0.02 0.004 0.03 5 0.57 1.5 1.5 0.01 0.004 0.03 6 0.50 0.5 1.5 0.030.004 1.0 7 0.41 0.3 0.3 0.01 0.004 0.03 8 0.42 1.5 1.5 0.02 0.005 0.03Mo: 0.2 9 0.40 1.5 1.5 0.01 0.006 0.03 Ni: 0.2 10 0.41 1.5 1.5 0.020.006 0.03 Cu: 0.2 11 0.40 1.5 1.5 0.03 0.005 0.03 Cr: 0.2 12 0.41 1.51.5 0.01 0.006 0.03 Ti: 0.03 13 0.40 1.5 1.5 0.02 0.005 0.03 Nb: 0.03 140.41 1.5 1.5 0.02 0.006 0.03 V: 0.03 15 0.40 1.5 1.5 0.02 0.005 0.03 Ca:10 ppm

[0674] TABLE 16 Steel TM B γR F Others C_(γR) TS EI λ YR No. No. (%) (%)(%) (%) (%) (%) (Mpa) (%) (%) (%) 1 1 56 5 0 41 0 — 710 27 59 66 2 2 418 8 43 0 0.7 810 53 54 65 3 3 32 6 18 44 0 0.6 720 25 41 60 4 3 39 6 1745 0 1.5 850 56 55 64 5 4 32 7 21 48 0 1.7 910 55 55 65 6 5 25 4 22 49 01.9 900 48 56 61 7 6 31 6 18 45 0 1.8 890 49 59 60 8 7 36 5 5 58 0 0.8830 25 53 90 9 8 44 3 18 45 0 1.7 813 57 54 66 10 9 46 2 18 43 0 1.7 81056 52 64 11 10 45 3 15 45 0 1.6 820 53 55 63 12 11 43 4 17 44 0 1.7 82352 54 65 13 12 41 5 16 46 0 1.7 818 51 53 61 14 13 45 3 17 43 0 1.6 82050 56 62 15 14 45 2 15 45 0 1.5 825 51 53 61 16 15 42 6 17 43 0 1.6 82252 54 62 17 3 56 4 20 20 0 1.5 800 55 61 62 18 4 54 5 18 23 0 1.6 820 5464 63 19 5 53 5 20 22 0 1.4 815 54 63 62 20 4 22 5 10 40 23(P) 0.6 77028 32 60 21 4 35 0 20 40  5(M) 1.5 840 55 50 60

[0675] TABLE 17 Steel M B γR F C_(γR) TS El λ YR No. No. (%) (%) (%) (%)(%) (Mpa) (%) (%) (%) 22 2 23 3 0 74 — 850 22 43 52 23 3 0 4 12 84 1.4788 37 41 67 24 2 0 83 0 17 — 830 15 59 93

[0676] TABLE 18 Hot rolling Cold rolling Continuous annealing SRT FDTCR1 T CR2 Average CR CT Cold rolling T1 CR T2 No. ° C. ° C. ° C./s ° C.° C./s ° C./s ° C. rate % ° C. ° C./s ° C. Hot rolling → 1 1150 850 40700 40 20 200 — — — — Continuous annealing 2 1150 850 40 700 40 20 450 —— — — 3 1150 850 40 700 40 20 RT — — — — 4 1150 850 40 700 40 20 RT — —— — 5 1150 850 40 — — 40 550 — — — — 6 1150 850  5 — —  5 200 — — — — 71150 750 40 — — 40 200 — — — — 8 1150 850 40 700 40 20 200 — — — — 91150 850 40 700 40 20 200 — — — — 10 1150 850 40 700 40 20 200 — — — —11 1150 850 40 700 40 20 200 — — — — 12 1150 850 40 700 40 20 200 — — —— 13 1150 850 40 700 40 20 200 — — — — 14 1150 850 40 700 40 20 200 — —— — 15 1150 850 40 700 40 20 200 — — — — 16 1150 850 40 700 40 20 200 —— — — 17 1150 850 40 700 40 20 200 — — — — 18 1150 850 40 700 40 20 200— — — — 19 1150 850 40 700 40 20 200 — — — — 20 1150 850 40 700 40 20200 — — — — 21 1150 850 40 700 40 20 200 — — — — 22 1150 850 40 700 4020 200 — — — — 23 1150 850 40 700 40 20 200 — — — — Hot rolling →Plating 24 1150 850 40 700 40 20 200 — — — — 25 1150 750 40 — — 40 200 —— — — Hot rolling → Cold 26 1150 850 40 700 40 20 200 30 — — — rolling →Continuous 27 1150 850 40 700 40 20 200 60 — — — annealing 28 1150 75040 — — 40 200 30 — — — Hot rolling → Cold 29 1150 850  5 700 40 10 20040 — — — rolling → Plating 30 1150 750  5 — —  5 200 40 — — — Continuousannealing or plating T3 t3 Tq CR T4 t4 Zn→GA Desired ° C. sec ° C. °C./s ° C. sec ° C. structure Details of structure Hot rolling → 800 60700 25 400 10 — ◯ Continuous annealing 800 60 700 25 400 10 — X F + B800 60 — 25 400 10 — ◯ C_(γR: 1.3%) 800 60 700 25 400 10 — ◯C_(γR: 1.7%) 800 60 700 25 400 10 — X Conventional TRIP 800 60 700 25400 10 — X Conventional TRIP 800 60 700 25 400 10 — ◯ Rolling in twophase region 900 60 700 25 400 10 — X Conventional TRIP 850 60 700 25400 10 — ◯ 750 60 700 25 400 10 — ◯ 700 60 700 25 400 10 — ◯ 650 60 70025 400 10 — X γ-less 800 180 700 25 400 10 — ◯ 800 30 700 25 400 10 — ◯800  5 700 25 400 10 — X Insufficient tempering 800 60 700 20 400 10 — ◯800 30 700  3 400 10 — X P 800 60 700 25 450 10 — ◯ 800 60 700 25 350 10— ◯ 800 60 700 25 200 10 — X Austempering not performed 800 60 700 25 RT— — X Austempering not performed 800 60 700 25 400 100 — ◯ 800 60 700 25400 1 — ◯ Hot rolling → Plating 800 60 700 25 400 10 600 ◯ 800 60 700 25400 10 600 ◯ Rolling in two phase region Hot rolling → Cold 800 60 70025 400 10 — ◯ rolling → Continuous 800 60 700 25 400 10 — X Tempered Mnot obtained annealing 800 60 700 25 400 10 — ◯ Hot rolling → Cold 80060 700 25 400 10 600 ◯ rolling → Plating 800 60 700 25 400 10 600 ◯

[0677] TABLE 19 Hot rolling Cold rolling Continuous annealing SRT FDTCR1 T CR2 Average CR CT Cold rolling T1 CR T2 No. ° C. ° C. ° C./s ° C.° C./s ° C./s ° C. rate % ° C. ° C./s ° C. Hot rolling → 31 1150 850 40— — 40 550 60 900 20 RT rolling → First 32 1150 850 40 — — 40 550 60 85020 RT continuous annealing 33 1150 850 40 — — 40 550 60 800 20 RT →Second continuous 34 1150 850 40 — — 40 550 60 750 20 RT annealing 351150 850 40 — — 40 550 60 700 20 RT 36 1150 850 40 — — 40 550 60 800 50RT 37 1150 850 40 — — 40 550 60 800 10 RT 38 1150 850 40 — — 40 550 60800 +E,usn  5 RT 39 1150 850 40 — — 40 550 60 800 20 200 40 1150 850 40— — 40 550 60 800 20 RT 41 1150 850 40 — — 40 550 60 800 20 RT 42 1150850 40 — — 40 550 60 800 20 RT 43 1150 850 40 — — 40 550 60 800 20 RT 441150 850 40 — — 40 550 60 800 20 RT 45 1150 850 40 — — 40 550 60 800 20RT 46 1150 850 40 — — 40 550 60 800 20 RT 47 1150 850 40 — — 40 550 60800 20 RT 48 1150 850 40 — — 40 550 60 800 20 RT 49 1150 850 40 — — 40550 60 800 20 RT 50 1150 850 40 — — 40 550 60 800 20 RT 51 1150 850 40 —— 40 550 60 800 20 RT 52 1150 850 40 — — 40 550 60 800 20 RT 53 1150 85040 — — 40 550 60 800 20 RT 54 1150 850 40 — — 40 550 60 800 20 RT 551150 850 40 — — 40 550 60 800 20 RT 56 1150 850 40 — — 40 550 60 800 20RT Hot rolling → Cold 57 1150 850 40 — — 40 550 50 800 20 RT rolling →First continuous annealing → Plating Continuous annealing or plating T3t3 Tq CR T4 t4 Zn→GA Desired ° C. sec ° C. ° C./s ° C. sec ° C.structure Details of structure Hot rolling → Cold 800  60 700 25 400 10— ◯ rolling → First 800  60 700 25 400 10 — ◯ continuous 800  60 700 25400 10 — ◯ annealing → 800  60 700 25 400 10 — ◯ Second continuous 800 60 700 25 400 10 — X γ-less annealing 800  60 700 25 400 10 — ◯ 800  60700 25 400 10 — ◯ 800  60 700 25 400 10 — X F, P 800  60 700 25 400 10 —◯ 900  60 700 25 400 10 — X Conventional TRIP 850  60 700 25 400 10 — ◯750  60 700 25 400 10 — ◯ 700  60 700 25 400 10 — ◯ 650  60 700 25 40010 — X γ-less 800 1000 700 25 400 10 — X Tempered M not obtained 800 180 700 25 400 10 — ◯ 800  30 700 25 400 10 — ◯ 800   5 700 25 400 10 —X Insufficient tempering 800  60 700 20 400 10 — ◯ 800  30 700  3 400 10— X P 800  60 700 25 450 10 — ◯ 800  60 700 25 350 10 — ◯ 800  60 700 25200 10 — X Austempering not performed 800  60 700 25 RT — — XAustempering not performed 800  60 700 25 400 100 — ◯ 800  60 700 25 4001 — ◯ Hot rolling → Cold 800  60 700 25 400 10 600 ◯ rolling → Firstcontinuous annealing → Plating

[0678] TABLE 20 Steel TB B γR F Others C_(γR) TS EI λ YR No. No. (%) (%)(%) (%) (%) (%) (Mpa) (%) (%) (%) 1 1 47 5 9 48 0 — 760 18 60 71 2 2 425 9 44 0 0.5 770 15 49 71 3 2 37 8 8 47 0 1.2 790 38 57 65 4 3 29 6 1055 0 1.1 800 36 57 67 5 4 32 7 13 48 0 1.3 805 38 54 63 6 5 35 6 12 47 01.3 780 40 56 66 7 6 40 5 1 54 0 0.4 790 25 61 71 8 7 31 3 8 58 0 1.2790 37 55 66 9 8 41 2 9 48 0 1.4 795 36 52 68 10 9 50 3 7 40 0 1.4 80036 54 65 11 10 38 4 9 49 0 1.3 810 38 53 66 12 11 37 5 8 50 0 1.4 805 3755 68 13 12 28 3 9 60 0 1.3 790 39 49 65 14 13 39 2 8 51 0 1.2 795 38 5167 15 14 40 6 9 45 0 1.2 800 39 53 68 16 2 64 8 8 20 0 1.3 800 40 61 6517 3 64 6 10 20 0 1.4 810 39 60 67 18 4 59 7 13 21 0 1.5 820 41 62 63 193 41 4 10 28 17(P) — 770 29 43 68 20 4 32 0 13 50  5(M) 1.3 790 38 55 60

[0679] TABLE 21 Steel TM B γR F Others C_(γR) TS EI λ YR No. No. (%) (%)(%) (%) (%) (%) (Mpa) (%) (%) (%) 1 1 53 4 0 43 0 — 680 27 54 65 2 2 435 8 44 0 0.5 690 23 45 65 3 3 43 5 13 39 0 1.5 805 56 57 67 4 4 41 6 1241 0 1.4 880 57 54 66 5 5 40 5 12 44 0 1.5 880 56 53 65 6 6 42 4 13 41 01.5 800 58 55 65 7 7 37 4 1 58 0 1.5 830 25 60 90 8 8 45 4 11 40 0 1.5790 58 56 66 9 9 45 5 12 38 0 1.6 800 55 57 65 10 10 45 3 13 39 0 1.5810 56 55 64 11 11 44 4 12 40 0 1.6 790 54 54 65 12 12 40 5 14 41 0 1.5805 56 56 67 13 13 41 4 13 42 0 1.4 800 55 56 65 14 14 38 7 12 43 0 1.4806 54 55 64 15 15 39 5 14 44 0 1.4 803 54 60 65 16 3 63 7 10 20 0 1.5780 55 60 65 17 4 60 5 13 22 0 1.5 810 56 63 66 18 6 59 6 12 23 0 1.4870 57 60 65 19 4 23 6 10 40 21(P) 1.5 780 25 43 73 20 4 40 0 13 40 7(M) 1.5 880 56 52 50

[0680] TABLE 22 Steel M B γR F C_(γr) TS EI λ YR No. No. (%) (%) (%) (%)(%) (Mpa) (%) (%) (%) 21 3 0 4 12 84 1.4 788 37 41 67 22 2 0 83 0 17 —830 15 59 93

[0681] TABLE 23 Hot rolling Cold rolling Continuous annealing SRT FDTCR1 T CR2 Average CR CT Cold rolling T1 CR T2 No. ° C. ° C. ° C./s ° C.° C./s ° C./s ° C. rate % ° C. ° C./s ° C. Hot rolling → 1 1150 850 40700 40 20 450 — — — — Continuous 2 1150 850 40 700 40 20 450 — — — —annealing 3 1150 850 40 700 40 20 450 — — — — 4 1150 850 40 700 40 20 RT— — — — 5 1150 850 40 — — 40 550 — — — — 6 1150 850  5 — —  5 450 — — —— 7 1150 750 40 — — 40 450 — — — — 8 1150 850 40 700 40 20 450 — — — — 91150 850 40 700 40 20 450 — — — — 10 1150 850 40 700 40 20 450 — — — —11 1150 850 40 700 40 20 450 — — — — 12 1150 850 40 700 40 20 450 — — —— 13 1150 850 40 700 40 20 450 — — — — 14 1150 850 40 700 40 20 450 — —— — 15 1150 850 40 700 40 20 450 — — — — 16 1150 850 40 700 40 20 450 —— — — 17 1150 850 40 700 40 20 450 — — — — 18 1150 850 40 700 40 20 450— — — — 19 1150 850 40 700 40 20 450 — — — — 20 1150 850 40 700 40 20450 — — — — 21 1150 850 40 700 40 20 450 — — — — 22 1150 850 40 700 4020 450 — — — — 23 1150 850 40 700 40 20 450 — — — — Hot rolling →Plating 24 1150 850 40 700 40 20 450 — — — — 25 1150 750 40 — — 40 450 —— — — Hot rolling → Cold 26 1150 850 40 700 40 20 450 30 — — — rolling →Continuous 27 1150 850 40 700 40 20 450 60 — — — annealing 28 1150 75040 — — 40 450 30 — — — Hot rolling → Cold 29 1150 850  5 700 40 10 45040 — — — rolling → Plating 30 1150 750  5 — —  5 450 40 — — — Continuousannealing or plating T3 t3 Tq CR T4 t4 Zn→GA Desired ° C. sec ° C. °C./s ° C. sec ° C. structure Details of structure Hot rolling → 800  60700 25 400 10 — ◯ Continuous 800  60 700 25 400 10 — ◯ C_(γR): 1.5%annealing 800  60 — 25 400 10 — ◯ C_(γR): 1.0% 800  60 700 25 400 10 — XF + tempered M 800  60 700 25 400 10 — X Conventional TRIP 800  60 70025 400 10 — X Conventional TRIP 800  60 700 25 400 10 — ◯ Rolling in twophase region 900  60 700 25 400 10 — X Conventional 850  60 700 25 40010 — ◯ 750  60 700 25 400 10 — ◯ 700  60 700 25 400 10 — ◯ 650  60 70025 400 10 — X γ-less 800 180 700 25 400 10 — ◯ 800  30 700 25 400 10 — ◯800  5 700 25 400 10 — X Insufficient tempering 800  60 700 20 400 10 —◯ 800  30 700  3 400 10 — X P 800  60 700 25 450 10 — ◯ 800  60 700 25350 10 — ◯ 800  60 700 25 200 10 — X Austempering not performed 800  60700 25 RT — — X Austempering not performed 800  60 700 25 400 100 — ◯800  60 700 25 400 1 — ◯ Hot rolling → Plating 800  60 700 25 400 10 600◯ 800  60 700 25 400 10 600 ◯ Rolling in two phase region Hot rolling →Cold 800  60 700 25 400 10 — ◯ rolling → Continuous 800  60 700 25 40010 — X Tempered B not obtained annealing 800  60 700 25 400 10 — ◯ Hotrolling → Cold 800  60 700 25 400 10 600 ◯ rolling → Plating 800  60 70025 400 10 600 ◯

[0682] TABLE 24 Hot rolling Cold rolling Continuous annealing SRT FDTCR1 T CR2 Average CR CT Cold rolling T1 CR T2 No. ° C. ° C. ° C./s ° C.° C./s ° C./s ° C. rate % ° C. ° C./s ° C. Hot rolling → 31 1150 850 40— — 40 550 60 900 20 450 Cold rolling → First 32 1150 850 40 — — 40 55060 850 20 450 continous annealing → 33 1150 850 40 — — 40 550 60 800 20450 Second continuous 34 1150 850 40 — — 40 550 60 750 20 450 annealing35 1150 850 40 — — 40 550 60 700 20 450 36 1150 850 40 — — 40 550 60 80050 450 37 1150 850 40 — — 40 550 60 800 10 450 38 1150 850 40 — — 40 55060 800  5 450 39 1150 850 40 — — 40 550 60 800 20 400 40 1150 850 40 — —40 550 60 800 20 450 41 1150 850 40 — — 40 550 60 800 20 450 42 1150 85040 — — 40 550 60 800 20 450 43 1150 850 40 — — 40 550 60 800 20 450 441150 850 40 — — 40 550 60 800 20 450 45 1150 850 40 — — 40 550 60 800 20450 46 1150 850 40 — — 40 550 60 800 20 450 47 1150 850 40 — — 40 550 60800 20 450 48 1150 850 40 — — 40 550 60 800 20 450 49 1150 850 40 — — 40550 60 800 20 450 50 1150 850 40 — — 40 550 60 800 20 450 51 1150 850 40— — 40 550 60 800 20 450 52 1150 850 40 — — 40 550 60 800 20 450 53 1150850 40 — — 40 550 60 800 20 450 54 1150 850 40 — — 40 550 60 800 20 45055 1150 850 40 — — 40 550 60 800 20 450 56 1150 850 40 — — 40 550 60 80020 450 Hot rolling → Cold 57 1150 850 40 — — 40 550 50 800 20 450rolling → First continuous annealing → Plating Continuous annealing orplating T3 t3 Tq CR T4 t4 Zn→GA Desired ° C. sec ° C. ° C./s ° C. sec °C. structure Details of structure Hot rolling → 800  60 700 25 400 10 —X F + tempered M rolling → First 800  60 700 25 400 10 — ◯ continousannealing → 800  60 700 25 400 10 — ◯ Second continuous 800  60 700 25400 10 — ◯ annealing 800  60 700 25 400 10 — X γ-less 800  60 700 25 40010 — ◯ 800  60 700 25 400 10 — ◯ 800  60 700 25 400 10 — X F, P 800  60700 25 400 10 — ◯ 900  60 700 25 400 10 — X Conventional TRIP 850  60700 25 400 10 — ◯ 750  60 700 25 400 10 — ◯ 700  60 700 25 400 10 — ◯650  60 700 25 400 10 — X γ-less 800 100 700 25 400 10 — X Tempered Bnot obtained 800 180 700 25 400 10 — ◯ 800  30 700 25 400 10 — ◯ 800  5700 25 450 10 — X Insufficient tempering 800  60 700 20 400 10 — ◯ 800 30 700  3 400 10 — X P 800  60 700 25 450 10 — ◯ 800  60 700 25 350 10— ◯ 800  60 700 25 200 10 — X Austempering not performed 800  60 700 25RT — — X Austempering not performed 800  60 700 25 400 100 — ◯ 800  60700 25 400 1 — ◯ Hot rolling → Cold 800  60 700 25 400 10 600 ◯ rolling→ First continuous annealing → Plating

[0683] TABLE 25 No. C Si Mn P S Al Others 1 0.03 1.5 1.5 0.08 0.0050.035 2 0.09 1.5 1.5 0.09 0.005 0.035 3 0.15 1.5 1.5 0.07 0.006 0.035 40.20 1.5 1.5 0.06 0.004 0.035 5 0.15 0.3 0.3 0.07 0.004 0.035 6 0.15 1.51.5 0.08 0.005 0.035 Mo: 0.2 7 0.15 1.5 1.5 0.07 0.006 0.035 Ni: 0.2 80.15 1.5 1.5 0.06 0.006 0.035 Cu: 0.2 9 0.15 1.5 1.5 0.07 0.005 0.035Cr: 0.2 10 0.15 1.5 1.5 0.07 0.006 0.035 Ti: 0.03 11 0.15 1.5 1.5 0.060.005 0.035 Nb: 0.03 12 0.15 1.5 1.5 0.07 0.006 0.035 V: 0.03 13 0.151.5 1.5 0.06 0.005 0.035 Ca: 10 ppm

[0684] TABLE 26 Continuous annealing or plating Hot rolling Cold rollingContinuous annealing Tempering Zn→ Steel SRT FDT CR CT Cold rolling T1CR T2 Temp. Time T3 t3 Tq CR T4 t4 GA No. No. ° C. ° C. ° C./s ° C. rate% ° C. ° C./s ° C. ° C. sec ° C. sec ° C. ° C./s ° C. sec ° C. 1 1 1150850 40 550 50 850 20 RT — — 800 60 700 10 400 100 — 2 2 1150 850 40 55050 850 20 RT — — 800 60 700 10 400 100 — 3 2 1150 850 40 550 50 850 20RT 450 1000 800 60 700 10 400 100 — 4 3 1150 850 40 550 50 850 20 RT 4501000 800 60 700 10 400 100 — 5 4 1150 850 40 550 50 850 20 RT 450 1000800 60 700 10 400 100 — 6 5 1150 850 40 550 50 850 20 RT 450 1000 800 60700 10 400 100 — 7 6 1150 850 40 550 50 850 20 RT 450 1000 800 60 700 10400 100 — 8 7 1150 850 40 550 50 850 20 RT 450 1000 800 60 700 10 400100 — 9 8 1150 850 40 550 50 850 20 RT 450 1000 800 60 700 10 400 100 —10 9 1150 850 40 550 50 850 20 RT 450 1000 800 60 700 10 400 100 — 11 101150 850 40 550 50 850 20 RT 450 1000 800 60 700 10 400 100 — 12 11 1150850 40 550 50 850 20 RT 450 1000 800 60 700 10 400 100 — 13 12 1150 85040 550 50 850 20 RT 450 1000 800 60 700 10 400 100 — 14 13 1150 850 40550 50 850 20 RT 450 1000 800 60 700 10 400 100 — 15 3 1150 850 40 55050 850 20 RT 450 1000 800 60 700 1 400 100 —

[0685] TABLE 27 Steel F TM B γR Others C_(γR) TS EI λ σw/YP No. No. (%)(%) (%) (%) (%) (%) (S1/S) × 100 (Mpa) (%) (%) (%) 1 1 48 47 5 0 0 — —460 33 96 0.80 2 2 44 42 5 9 0 1.4 17.5 610 26 54 0.67 3 2 47 37 8 8 01.4 8.4 590 38 72 0.83 4 3 55 29 6 10 0 1.3 10.2 750 40 59 0.87 5 4 4735 6 12 0 1.3 11.0 805 33 71 0.77 6 5 54 40 5 1 0 0.5 — 680 25 62 0.89 76 50 39 3 8 0 1.3 13.1 960 25 67 0.72 8 7 46 41 2 11 0 1.3 7.2 795 32 680.80 9 8 38 50 3 9 0 1.4 3.8 800 32 71 0.79 10 9 39 44 5 12 0 1.3 6.6810 33 66 0.80 11 10 44 37 5 14 0 1.4 13.1 805 30 73 0.80 12 11 53 31 313 0 1.4 699 790 33 64 0.83 13 12 44 40 4 12 0 1.3 10.8 795 34 67 0.8114 13 39 47 3 11 0 1.3 3.6 800 32 73 0.82 15 3 30 39 6 8 17(P) 1.3 17.8730 28 43 0.67

[0686] TABLE 28 Steel M B γR F C_(γR) TS EI λ YR σw/YP No. No. (%) (%)(%) (%) (%) (S1/S) × 100 (Mpa) (%) (%) (%) (%) 20 2 23 3 0 74 — 18.6 85022 43 52 0.61 21 3 0 4 12 84 1.4 22.3 788 37 41 67 0.65 22 2 0 83 0 17 —— 830 15 59 93 0.88

[0687] TABLE 29 Hot rolling Tempered Continuous annealing or plating SRTFDT CR1 T CR2 Average CR CT Temp. Time T3 t3 Tq CR T4 t4 No. ° C. ° C. °C./s ° C. ° C./s ° C./s ° C. ° C. sec ° C. sec ° C. ° C./s ° C. sec Hot1 1150 850 40 — — 40 200 — — 800 60 700 10 400 100 rolling → 2 1150 85040 — — 40 200 450 1000 800 60 700 10 400 100 Continuous 3 1150 850 40700 40 20 200 — — 800 60 700 10 400 100 annealing 4 1150 850 40 700 4020 200 450 1000 800 60 700 10 400 100 5 1150 850 40 — — 40 450 — — 80060 700 10 400 100 6 1150 850 40 700 40 20 450 — — 800 60 700 10 400 100

[0688] TABLE 30 Second Base phase phase structure TS E1 λ No. structure(S1/S) × 100 (MPa) (%) (%) FL/YP 1 TM 33.1 750 40 44 0.72 2 TM 7.5 75040 64 0.86 3 F + TM 27.8 750 40 45 0.73 4 F + TM 11.3 750 40 63 0.89 5TB 1.8 750 40 69 0.87 6 F + TB 6.2 750 40 64 0.83

[0689] TABLE 31 Hot rolling Cold rolling Continuous annealing Steel SRTFDT CR CT Cold rolling T1 CR T2 No. No. ° C. ° C. ° C./s ° C. rate % °C. ° C./s ° C. Hot rolling → 1 1 1150 850 40 550 50 850 20 RT Coldrolling → 2 1 1150 850 40 550 50 850 20 RT First continuous 3 2 1150 85040 550 50 850 20 RT annealing → 4 2 1150 850 40 550 50 850 20 RT Second5 3 1150 850 40 550 50 850 20 RT continuous 6 3 1150 850 40 550 50 85020 RT annealing 7 3 1150 850 40 550 50 850 20 RT 8 3 1150 850 40 550 50850 20 RT 9 3 1150 850 40 550 50 850 20 RT 10 3 1150 850 40 550 50 85020 RT 11 3 1150 850 40 550 50 850 20 450 12 3 1150 850 40 550 50 900 20RT 13 3 1150 850 40 550 50 900 20 RT 14 3 1150 850 40 550 50 900 20 45015 4 1150 850 40 550 50 850 20 RT 16 4 1150 850 40 550 50 850 20 RT 17 51150 850 40 550 50 850 20 RT 18 5 1150 850 40 550 50 850 20 RT 19 6 1150850 40 550 50 850 20 RT 20 6 1150 850 40 550 50 850 20 RT 21 7 1150 85040 550 50 850 20 RT 22 7 1150 850 40 550 50 850 20 RT 23 8 1150 850 40550 50 850 20 RT 24 8 1150 850 40 550 50 850 20 RT 25 9 1150 850 40 55050 850 20 RT 26 9 1150 850 40 550 50 850 20 RT 27 10 1150 850 40 550 50850 20 RT 28 10 1150 850 40 550 50 850 20 RT 29 11 1150 850 40 550 50850 20 RT 30 11 1150 850 40 550 50 850 20 RT 31 12 1150 850 40 550 50850 20 RT 32 12 1150 850 40 550 50 850 20 RT 33 13 1150 850 40 550 50850 20 RT 34 13 1150 850 40 550 50 850 20 RT Tempered Continuousannealing or plating Temp. Time T3 t3 Tq CR T4 t4 Zn→GA ° C. sec ° C.sec ° C. ° C./s ° C. sec ° C. Hot rolling → — — 800 60 700 10 400 100 —Cold rolling → 450 1000 800 60 700 10 400 100 — First continuous — — 80060 700 10 400 100 — annealing → 450 1000 800 60 700 10 400 100 — Second— — 800 60 700 10 400 100 — continuous 300 1000 800 60 700 10 400 100 —annealing 450 1000 800 60 700 10 400 100 — 600 1000 800 60 700 10 400100 — 600 3600 800 60 700 10 400 100 — 750 3600 800 60 700 10 400 100 —— — 800 60 700 10 400 100 — — — 800 60 700 10 400 100 — 450 1000 800 60700 10 400 100 — — — 800 60 700 10 400 100 — — — 800 60 700 10 400 100 —450 1000 800 60 700 10 400 100 — — — 800 60 700 10 400 100 — 450 1000800 60 700 10 400 100 — — — 800 60 700 10 400 100 — 450 1000 800 60 70010 400 100 — — — 800 60 700 10 400 100 — 450 1000 800 60 700 10 400 100— — — 800 60 700 10 400 100 — 450 1000 800 60 700 10 400 100 — — — 80060 700 10 400 100 — 450 1000 800 60 700 10 400 100 — — — 800 60 700 10400 100 — 450 1000 800 60 700 10 400 100 — — — 800 60 700 10 400 100 —450 1000 800 60 700 10 400 100 — — — 800 60 700 10 400 100 — 450 1000800 60 700 10 400 100 — — — 800 60 700 10 400 100 — 450 1000 800 60 70010 400 100 —

[0690] TABLE 32 Second Base phase phase structure TS E1 λ No. structure(S1/S) × 100 (MPa) (%) (%) FL/YP 1 F + TM — 460 33 96 0.80 2 F + TM —460 33 98 0.81 3 F + TM 30.3 590 38 57 0.72 4 F + TM 8.4 590 38 72 0.835 F + TM 28.5 750 40 45 0.78 6 F + TM 23.3 750 40 42 0.74 7 F + TM 10.2750 40 59 0.87 8 F + TM 7.8 750 40 63 0.85 9 F + TM 8.5 750 40 66 0.8610 F + TM 25.7 750 40 46 0.76 11 F + TB 33.1 750 40 61 0.86 12 TM 30.3750 40 43 0.74 13 TM 8.4 750 40 64 0.88 14 TB 27.8 750 40 59 0.86 15 F +TM 25.3 805 33 54 0.69 16 F + TM 11.0 805 33 71 0.77 17 F + TM — 680 2561 0.88 18 F + TM — 680 25 62 0.89 19 F + TM 35.8 960 25 55 0.65 20 F +TM 13.1 960 25 67 0.72 21 F + TM 27.4 795 32 52 0.71 22 F + TM 7.2 79532 68 0.80 23 F + TM 28.0 800 32 54 0.69 24 F + TM 3.8 800 32 71 0.79 25F + TM 25.4 810 33 53 0.71 26 F + TM 6.6 810 33 66 0.80 27 F + TM 27.1805 30 55 0.72 28 F + TM 13.1 805 30 73 0.80 29 F + TM 33.1 790 33 490.71 30 F + TM 8.5 790 33 64 0.83 31 F + TM 24.8 795 34 51 0.72 32 F +TM 10.8 795 34 67 0.81 33 F + TM 28.7 800 32 53 0.74 34 F + TM 3.6 80032 73 0.82

[0691] TABLE 33 Hot rolling Cold rolling Continuous annealing Steel SRTFDT CR CT Cold rolling T1 CR T2 No. No. ° C. ° C. ° C./s ° C. rate % °C. ° C./s ° C. Hot rolling → 1 3 1150 850 40 550 50 850 20 RT Coldrolling → 2 3 1150 850 40 550 50 850 20 RT First continuous 3 3 1150 85040 550 50 850 20 450 annealing → 4 3 1150 850 40 550 50 900 20 RT Second5 3 1150 850 40 550 50 900 20 RT continuous 6 3 1150 850 40 550 50 90020 450 annealing Tempered Continuous annealing or plating Temp. Time T3t3 Tq CR T4 t4 Zn→GA ° C. sec ° C. sec ° C. ° C./s ° C. sec ° C. Hotrolling → — — 800 60 700 10 400 100 — Cold rolling → 450 1000 800 60 70010 400 100 — First continuous — — 800 60 700 10 400 100 — annealing → —— 800 60 700 10 400 100 — Second 450 1000 800 60 700 10 400 100 —continuous — — 800 60 700 10 400 100 — annealing

[0692] TABLE 34 Second Base phase phase structure TS E1 λ No. structure(S1/S) (MPa) (%) (%) FL/YP 1 F + TM 23.6 750 40 44 0.73 2 F + TM 3.5 75040 56 0.81 3 F + TB 28.0 750 40 62 0.80 4 TM 30.3 750 40 48 0.74 5 TM7.2 750 40 62 0.84 6 TB 27.5 750 40 67 0.85

[0693] TABLE 35 No. C Si Mn P S Al Others 1 0.03 1.5 1.5 0.08 0.0050.035 2 0.09 1.5 1.5 0.09 0.005 0.035 3 0.15 1.5 1.5 0.07 0.08 0.035 40.20 1.5 1.5 0.06 0.004 0.035 5 0.15 0.3 0.3 0.07 0.004 0.035 6 0.15 1.51.5 0.08 0.005 0.035 Mo: 0.2 7 0.15 1.5 1.5 0.07 0.006 0.035 Ni: 0.2 80.15 1.5 1.5 0.06 0.006 0.035 Cu: 0.2 9 0.15 1.5 1.5 0.07 0.005 0.035Cr: 0.2 10 0.15 1.5 1.5 0.07 0.006 0.035 Ti: 0.03 11 0.15 1.5 1.5 0.060.005 0.035 Nb: 0.03 12 0.15 1.5 1.5 0.07 0.006 0.035 V: 0.03 13 0.151.5 1.5 0.06 0.005 0.035 Ca: 10 ppm

[0694] TABLE 36 Hot rolling Cold rolling Continuous annealing Continuousannealing or plating Steel SRT FDT CR CT Cold T1 CR T2 T3 t3 Tq CR T4 t4Zn→GA No. No. ° C. ° C. ° C./s ° C. rolling rate % ° C. ° C./s ° C. ° C.sec ° C. ° C./s ° C. sec ° C. 1 1 1050 850 40 550 50 850 20 RT 800 60700 10 400 100 — 2 2 1050 850 40 550 50 850 20 RT 800 60 700 10 400 100— 3 3 1050 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 4 4 1050 85040 550 50 850 20 RT 800 60 700 10 400 100 — 5 5 1050 850 40 550 50 85020 RT 800 60 700 10 400 100 — 6 6 1050 850 40 550 50 850 20 RT 800 60700 10 400 100 — 7 7 1050 850 40 550 50 850 20 RT 800 60 700 10 400 100— 8 8 1050 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 9 9 1050 85040 550 50 850 20 RT 800 60 700 10 400 100 — 10 10 1050 850 40 550 50 85020 RT 800 60 700 10 400 100 — 11 11 1050 850 40 550 50 850 20 RT 800 60700 10 400 100 — 12 12 1050 850 40 550 50 850 20 RT 800 60 700 10 400100 — 13 13 1050 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 14 31050 850 40 550 50 850 20 RT 800 60 700 1 400 100 — 15 3 950 850 40 55050 850 20 RT 800 60 700 1 400 100 — 16 3 975 850 40 550 50 850 20 RT 80060 700 1 400 100 — 17 3 1000 850 40 550 50 850 20 RT 800 60 700 1 400100 — 18 3 1025 850 40 550 50 850 20 RT 800 60 700 1 400 100 — 19 3 1075850 40 550 50 850 20 RT 800 60 700 1 400 100 — 20 3 1100 850 40 550 50850 20 RT 800 60 700 1 400 100 —

[0695] TABLE 37 Steel F TM B γR Others C_(γR) TS EI λ BH2 BH10 No. No.(%) (%) (%) (%) (%) (%) (MPa) (%) (%) (MPa) (MPa) 1 1 48 47 5 0 0 — 46534 96 5 0 2 2 44 42 5 9 0 1.4 610 26 54 80 45 3 3 55 29 6 10 0 1.3 76041 59 85 50 4 4 47 35 6 12 0 1.3 810 33 71 95 55 5 5 54 40 5 1 0 0.5 68524 62 15 0 6 6 50 39 3 8 0 1.3 960 26 67 85 50 7 7 46 41 2 11 0 1.3 80032 68 85 45 8 8 38 50 3 9 0 1.4 800 33 71 80 50 9 9 39 44 5 12 0 1.3 81033 66 90 55 10 10 44 37 5 14 0 1.4 805 31 73 90 50 11 11 53 31 3 13 01.4 795 33 64 80 45 12 12 44 40 4 12 0 1.3 795 35 67 80 45 13 13 39 47 311 0 1.3 800 32 73 85 45 14 3 30 39 6 8 17(P) 1.3 735 27 43 15 0 15 3 5034 4 12 0 1.3 770 40 37 100 60 16 3 53 31 5 11 0 1.3 765 39 60 95 60 173 56 27 7 10 0 1.4 765 40 57 95 55 18 3 53 33 3 11 0 1.3 760 40 62 90 5519 3 53 33 4 10 0 1.3 755 39 63 85 45 20 3 52 32 5 11 0 1.3 760 40 65 8045

[0696] TABLE 38 Hot rolling Cold rolling Continuous annealing Continuousannealing or plating Steel SRT FDT CR CT Cold T1 CR T2 T3 t3 Tq CR T4 t4Zn→GA No. No. ° C. ° C. ° C./s ° C. rolling rate % ° C. ° C./s ° C. ° C.sec ° C. ° C./s ° C. sec ° C. 1 1 1050 850 40 550 50 850 20 RT 800 60700 10 400 100 — 2 2 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100— 3 3 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 4 4 1150 85040 550 50 850 20 RT 800 60 700 10 400 100 — 5 5 1150 850 40 550 50 85020 RT 800 60 700 10 400 100 — 6 6 1150 850 40 550 50 850 20 RT 800 60700 10 400 100 — 7 7 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100— 8 8 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 9 9 1150 85040 550 50 850 20 RT 800 60 700 10 400 100 — 10 10 1150 850 40 550 50 85020 RT 800 60 700 10 400 100 — 11 11 1150 850 40 550 50 850 20 RT 800 60700 10 400 100 — 12 12 1150 850 40 550 50 850 20 RT 800 60 700 10 400100 — 13 13 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 14 31150 850 40 550 50 850 20 RT 800 60 700  1 400 100 — 15 3 925 850 40 55050 850 20 RT 800 60 700  1 400 100 — 16 3 1125 850 40 550 50 850 20 RT800 60 700  1 400 100 — 17 3 1175 850 40 550 50 850 20 RT 800 60 700  1400 100 — 18 3 1200 850 40 550 50 850 20 RT 800 60 700  1 400 100 —

[0697] TABLE 39 Steel F TM B γR Others C_(γR) TS EI λ BH2 BH10 No. No.(%) (%) (%) (%) (%) (%) (MPa) (%) (%) (MPa) (MPa) 1 1 48 47 5 0 0 — 46033 96 3 0 2 2 44 42 5 9 0 1.4 610 26 54 75 15 3 3 53 31 6 10 0 1.3 76041 59 85 52 4 4 47 35 6 12 0 1.3 805 33 71 95 40 5 5 54 40 5 1 0 0.5 68025 62 20 0 6 6 50 39 3 8 0 1.3 960 25 67 85 20 7 7 46 41 2 11 0 1.3 79532 68 80 15 8 8 38 50 3 9 0 1.4 800 32 71 80 10 9 9 39 44 5 12 0 1.3 81033 66 85 20 10 10 44 37 5 14 0 1.4 805 30 73 90 20 11 11 53 31 3 13 01.3 790 33 64 80 20 12 12 44 40 4 12 0 1.3 795 34 67 75 15 13 13 39 47 311 0 1.3 800 32 73 85 25 14 3 30 39 6 8 17(P) 1.3 730 28 43 15 0 15 3 8010 5 5 0 1.3 730 34 35 30 10 16 3 54 31 5 10 0 1.4 755 39 57 80 35 17 352 33 4 11 0 1.3 750 39 60 75 35 18 3 55 31 5 9 0 1.4 740 41 62 70 30

[0698] TABLE 40 Steel M B γR F C_(γr) TS EI λ BH2 BH10 No. No. (%) (%)(%) (%) (%) (Mpa) (%) (%) (MPa) (MPa) 1 2 23 3 0 74 — 850 22 43 40 10 23 0 4 12 84 1.4 788 37 41 55 15 3 2 0 83 0 17 — 830 15 59 10 0

[0699] TABLE 41 Hot rolling Continuous annealing or plating SRT FDT CR1T CR2 Average CR CT T3 t3 Tq CR T4 t4 No. ° C. ° C. ° C./s ° C. ° C./s °C./s ° C. ° C. sec ° C. ° C./s ° C. sec Hot rolling → 1 1050 850 40 — —40 200 800 60 700 10 400 100 Continuous 2 1050 850 40 700 40 20 200 80060 700 10 400 100 annealing 3 1050 850 40 — — 40 450 800 60 700 10 400100 4 1050 850 40 700 40 20 450 800 60 700 10 400 100 Hot rolling → 51150 850 40 — — 40 200 800 60 700 10 400 100 Continuous 6 1150 850 40700 40 20 200 800 60 700 10 400 100 annealing 7 1150 850 40 — — 40 450800 60 700 10 400 100 8 1150 850 40 700 40 20 450 800 60 700 10 400 100

[0700] TABLE 42 Base phase TS λ BH2 BH10 No. structure (MPa) E1 (%)(MPa) (MPa) 1 TM 755 40 44 100 55 2 F + TM 755 40 45 85 45 3 TB 755 4069 80 45 4 F + TB 755 40 64 80 45 5 TM 750 40 44 95 30 6 F + TM 750 4045 80 25 7 TB 750 40 69 80 20 8 F + TB 750 40 64 75 15

[0701] TABLE 43 Continuous Continuous annealing or plating Hot rollingCold rolling annealing Zn→ Steel SRT FDT CR CT Cold rolling T1 CR T2 T3t3 Tq CR T4 t4 GA No. No. ° C. ° C. ° C./s ° C. rate % ° C. ° C./s ° C.° C. sec ° C. ° C./s ° C. sec ° C. Hot rolling → 1 1 1050 850 40 550 50850 20 RT 800 60 700 10 400 100 — Cold rolling → 2 2 1050 850 40 550 50850 20 RT 800 60 700 10 400 100 — First continuous 3 3 1050 850 40 55050 850 20 RT 800 60 700 10 400 100 — annealing → 4 3 1050 850 40 550 50850 20 450 800 60 700 10 400 100 — Second 5 3 1050 850 40 550 50 900 20RT 800 60 700 10 400 100 — continuous 6 3 1050 850 40 550 50 900 20 450800 60 700 10 400 100 — annealing 7 4 1050 850 40 550 50 850 20 RT 80060 700 10 400 100 — 8 5 1050 850 40 550 50 850 20 RT 800 60 700 10 400100 — 9 6 1050 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 10 7 1050850 40 550 50 850 20 RT 800 60 700 10 400 100 — 11 8 1050 850 40 550 50850 20 RT 800 60 700 10 400 100 — 12 9 1050 850 40 550 50 850 20 RT 80060 700 10 400 100 — 13 10 1050 850 40 550 50 850 20 RT 800 60 700 10 400100 — 14 11 1050 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 15 121050 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 16 13 1050 850 40550 50 850 20 RT 800 60 700 10 400 100 —

[0702] TABLE 44 Base phase TS E1 λ BH2 BH10 No. structure (MPa) (%) (%)(MPa) (MPa) 1 F + TM 465 32 96 10  0 2 F + TM 590 39 62 75 45 3 F + TM755 40 51 85 50 4 F + TB 750 41 61 85 55 5 TM 750 40 48 80 45 6 TB 75541 59 85 55 7 F + TM 805 33 59 100  65 8 F + TM 680 26 66 20  5 9 F + TM965 26 55 80 45 10 F + TM 795 32 57 80 45 11 F + TM 800 33 59 85 50 12F + TM 815 33 58 75 45 13 F + TM 805 31 55 85 50 14 F + TM 795 34 49 8545 15 F + TM 795 35 56 85 50 16 F + TM 805 32 58 80 45

[0703] TABLE 45 Continuous Continuous annealing or plating Hot rollingCold rolling annealing Zn→ Steel SRT FDT CR CT Cold rolling T1 CR T2 T3t3 Tq CR T4 t4 GA No. No. ° C. ° C. ° C./s ° C. rate % ° C. ° C./s ° C.° C. sec ° C. ° C./s ° C. sec ° C. Hot rolling → 1 3 1050 850 40 550 50850 20 RT 800 60 700 10 400 100 600 Cold rolling → 2 3 1050 850 40 55050 850 20 450 800 60 700 10 400 100 600 First continuous 3 3 1050 850 40550 50 900 20 RT 800 60 700 10 400 100 600 annealing → 4 3 1050 850 40550 50 900 20 450 800 60 700 10 400 100 600 Second continuous annealing

[0704] TABLE 46 Base phase TS E1 λ BH2 BH10 No. structure (MPa) (%) (%)(MPa) (MPa) 1 F + TM 755 40 44 85 50 2 F + TB 755 40 62 75 45 3 TM 75540 48 105  60 4 TB 755 40 67 75 45

[0705] TABLE 47 Continuous Continuous annealing or plating Hot rollingCold rolling annealing Zn→ Steel SRT FDT CR CT Cold rolling T1 CR T2 T3t3 Tq CR T4 t4 GA No. No. ° C. ° C. ° C./s ° C. rate % ° C. ° C./s ° C.° C. sec ° C. ° C./s ° C. sec ° C. Hot rolling → 1 1 1150 850 40 550 50850 20 RT 800 60 700 11 400 100 — Cold rolling → 2 2 1150 850 40 550 50850 20 RT 800 60 700 10 400 100 — First continuous 3 3 1150 850 40 55050 850 20 RT 800 60 700 10 400 100 — annealing → 4 3 1150 850 40 550 50850 20 450 800 60 700 10 400 100 — Second 5 3 1150 850 40 550 50 900 20RT 800 60 700 10 400 100 — continuous 6 3 1150 850 40 550 50 900 20 450800 60 700 10 400 100 — annealing 7 4 1150 850 40 550 50 850 20 RT 80060 700 10 400 100 — 8 5 1150 850 40 550 50 850 20 RT 800 60 700 10 400100 — 9 6 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 10 7 1150850 40 550 50 850 20 RT 800 60 700 10 400 100 — 11 8 1150 850 40 550 50850 20 RT 800 60 700 10 400 100 — 12 9 1150 850 40 550 50 850 20 RT 80060 700 10 400 100 — 13 10 1150 850 40 550 50 850 20 RT 800 60 700 10 400100 — 14 11 1150 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 15 121150 850 40 550 50 850 20 RT 800 60 700 10 400 100 — 16 13 1150 850 40550 50 850 20 RT 800 60 700 10 400 100 —

[0706] TABLE 48 Base phase TS E1 λ BH2 BH10 No. structure (MPa) (%) (%)(MPa) (MPa) 1 F + TM 460 33 96 10  0 2 F + TM 590 38 57 70 20 3 F + TM750 40 46 85 25 4 F + TB 750 40 61 80 25 5 TM 750 40 43 80 25 6 TB 75040 59 75 25 7 F + TM 805 33 54 95 35 8 F + TM 680 25 61 85 20 9 F + TM960 25 55 80 25 10 F + TM 795 32 52 75 25 11 F + TM 800 32 54 85 20 12F + TM 810 33 53 70 25 13 F + TM 805 30 55 75 30 14 F + TM 790 33 49 8030 15 F + TM 795 34 51 85 25 16 F + TM 800 32 53 75 30

[0707] TABLE 49 Continuous Continuous annealing or plating Hot rollingCold rolling annealing Zn→ Steel SRT FDT CR CT Cold rolling T1 CR T2 T3t3 Tq CR T4 t4 GA No. No. ° C. ° C. ° C./s ° C. rate % ° C. ° C./s ° C.° C. sec ° C. ° C./s ° C. sec ° C. Hot rolling → 1 3 1150 850 40 550 50850 20 RT 800 60 700 10 400 100 600 Cold rolling → 2 3 1150 850 40 55050 850 20 450 800 60 700 10 400 100 600 First continuous 3 3 1150 850 40550 50 900 20 RT 800 60 700 10 400 100 600 annealing → 4 3 1150 850 40550 50 900 20 450 800 60 700 10 400 100 600 Second continuous annealing

[0708] TABLE 50 Base phase TS E1 λ BH2 BH10 No. structure (MPa) (%) (%)(MPa) (MPa) 1 F + TM 750 40 44 80 25 2 F + TB 750 40 62 70 15 3 TM 75040 48 95 35 4 TB 750 40 67 70 20

1. A high strength steel sheet superior in formability, (1) containingthe following chemical components in mass %: C: 0.06 to 0.6% Si+Al: 0.5to 3% Mn: 0.5 to 3% P: 0.15% or less (not including 0%) S: 0.02% or less(not including 0%), and (2) having a structure comprising: (2-1) a basephase structure, the base phase structure being tempered martensite ortempered bainite and accounting for 50% or more in terms of a spacefactor relative to the whole structure, or the base structure comprisingtempered martensite or tempered bainite which accounts for 15% or morein terms of a space factor relative to the whole structure and furthercomprising ferrite, the tempered martensite or the tempered bainitehaving a hardness which satisfies the relation of: Vickers hardness(Hv)≧500[C]+30[Si]+3[Mn]+50 where [ ] represents the content (mass %) ofeach element; and (2-2) a second phase structure comprising retainedaustenite which accounts for 3 to 30% in terms of a space factorrelative to the whole structure and optionally further comprisingbainite and/or martensite, the retained austenite having a Cconcentration (Cγ_(R)) of 0.8% or more.
 2. A high strength steel sheetaccording to claim 1, (1) containing the following chemical componentsin mass %: C: 0.06 to 0.25% Si+Al: 0.5 to 3% Mn: 0.5 to 3% P: 0.15% orless (not including 0%) S: 0.02% or less (not including 0%), and (2)wherein the second phase structure satisfies the following expression(1) to enhance the fatigue characteristic: (S1/S)×100≦20   (1) where Sstands for a total area of the second phase structure, and S1 stands fora total area of coarse second phase crystal grains (Sb) contained in thesecond phase structure, the Sb corresponding to three times or more aslarge as an average crystal grain area (Sm) of the second phasestructure.
 3. A high strength steel sheet according to claim 1, (1)containing the following chemical components in mass %: C: 0.06 to 0.25%Si+Al: 0.5 to 3% Mn: 0.5 to 3% P: 0.15% or less (not including 0%) S:0.02% or less (not including 0%), and (2) having such bake hardening(BH) characteristics after baking finish as satisfy the followingexpressions: BH (2%)≧70 MPa, and BH (10%)≧BH (2%)/2.
 4. A high strengthsteel sheet according to claim 1, wherein the retained austenite is in alath form.
 5. A high strength steel sheet according to claim 1, whereinthe content of the ferrite is 5 to 60% in terms of a space factorrelative to the whole structure.
 6. A high strength steel sheetaccording to claim 5, wherein the content of the ferrite is 5 to 30% interms of a space factor relative to the whole structure.
 7. A highstrength steel sheet according to claim 1, further containing at leastone of the following components in mass %: Mo: 1% or less (not including0%) Ni: 0.5% or less (not including 0%) Cu: 0.5% or less (not including0%) Cr: 1% or less (not including 0%).
 8. A high strength steel sheetaccording to claim 1, further containing at least one of the followingcomponents in mass %: Ti: 0.1% or less (not including 0%) Nb: 0.1% orless (not including 0%) V: 0.1% or less (not including 0%).
 9. A highstrength steel sheet according to claim 1, further containing thefollowing component(s) in mass %: Ca: 0.003% or less (not including 0%),and/or REM: 0.003% or less (not including 0%).
 10. A method of producingthe high strength steel sheet described in claim 1 wherein the basephase structure is tempered martensite or tempered bainite, the methodcomprising a hot rolling process and a continuous annealing process or aplating process, the hot rolling process comprising a step ofterminating finish rolling at a temperature of not lower than(A_(r3)−50)° C. and a step of cooling a resulting steel sheet to atemperature of not higher than Ms point or a temperature of not lowerthan Ms point and not higher than Bs point at an average cooling rate ofnot lower than 20° C./s and winding up the steel sheet, the continuousannealing process or the plating process comprising a step of holdingthe steel sheet in a heated state at a temperature of not lower than A₁point and not higher than A₃ point for 10 to 600 seconds, a step ofcooling the steel sheet to a temperature of not lower than 300° C. andnot higher than 480° C. at an average cooling rate of not lower than 3°C./s, and a step of holding the steel sheet in said temperature rangefor 1 second or more.
 11. A method of producing the high strength steeldescribed in claim 1 wherein the base phase structure is temperedmartensite or tempered bainite, the method comprising a hot rollingprocess, a cold rolling process, a first continuous annealing process,and a second continuous annealing process or a plating process, thecontinuous annealing process comprising a step of holding a resultingsteel sheet in a heated state at a temperature of not lower than A₃point and a step of cooling the steel sheet to a temperature of nothigher than Ms point or a temperature of not lower than Ms point and nothigher than Bs point at an average cooling rate of not lower than 20°C./s, the second continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet insaid temperature range for 1 second or more.
 12. A method of producingthe high strength steel sheet described in claim 1 wherein the basephase structure comprises tempered martensite and ferrite or comprisestempered bainite and ferrite, the method comprising a hot rollingprocess and a continuous annealing process or a plating process, the hotrolling process comprising a step of terminating finish rolling at atemperature of not lower than (A_(r3)−50)° C. and a step of cooling aresulting steel sheet to a temperature of not higher than Ms point or atemperature of not lower than Ms point and not higher than Bs point atan average cooling rate of not lower than 10° C./s and winding up thesteel sheet, the continuous annealing process or the plating processcomprising a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steep sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet insaid temperature range for 1 second or more.
 13. The method of claim 12,wherein the hot rolling process comprises a step of terminating finishrolling at a temperature of not lower than (A_(r3)−50)° C., a step ofcooling the steel sheet to a temperature in the range of 700±100° C. atan average cooling rate of not lower than 30° C./s, a step of coolingthe steel sheet with air in said temperature range for 1 to 30 seconds,and a step of subsequently cooling the steel sheet to a temperature ofnot higher than Ms point or a temperature of not lower than Ms point andnot higher than Bs point at an average cooling rate of not lower than30° C./s and winding up the steel sheet.
 14. The method of claim 12,wherein the continuous annealing process comprises a step of holding thesteel sheet in a heated state at a temperature of not lower than A₁point and not higher than A₃ point for 10 to 600 seconds, a step ofcooling the steel sheet to a temperature of (A₁ point to 60° C.) at anaverage cooling rate of not higher than 15° C./s, a step of cooling thesteel sheet to a temperature of not lower than 300° C. and not higherthan 480° C. at an average cooling rate of not lower than 20° C./s, anda step of holding the steel sheet in said temperature range for 1 secondor more.
 15. A method of producing the high strength steel described inclaim 1 wherein the base phase structure comprises tempered martensiteand ferrite or comprises tempered bainite and ferrite, the methodcomprising a hot rolling process, a cold rolling process, a firstcontinuous annealing process, a tempering process, and a secondcontinuous annealing process or a plating process, the first continuousannealing process comprising a step of holding a resulting steel sheetin a heated state at a temperature of not lower than A₁ point and nothigher than A₃ point and a step of cooling the steel sheet to atemperature of not higher than Ms point or a temperature of not lowerthan Ms point and not higher than Bs point at an average cooling rate ofnot lower than 10° C./s, the second continuous annealing process or theplating process comprising a step of holding the steel sheet in a heatedstate at a temperature of not lower than A₁ point and not higher than A₃point for 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in said temperature range for 1 second or more.
 16. Themethod of claim 15, wherein the second continuous annealing processcomprises a step of holding the steel sheet in a heated state at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature of(A₁ point to 600° C.) at an average cooling rate of not lower than 20°C./s, a step of cooling the steel sheet to a temperature of not lowerthan 300° C. and not higher than 480° C. at an average cooling rate ofnot lower than 20° C./s, and a step of holding the steel sheet in saidtemperature range for 1 second or more.
 17. A method of producing thehigh strength steel described in claim 2 wherein the base phasestructure is tempered martensite or tempered bainite, the methodcomprising a hot rolling process, a tempering process, and a continuousannealing process or a plating process, the hot rolling processcomprising a step of terminating finish rolling at a temperature of notlower than (A_(r3)−50)° C. and a step of cooling a resulting steel sheetto a temperature of not higher than Ms point or a temperature of notlower than Ms point and not higher than Bs point at an average coolingrate of not lower than 20° C./s, the tempering process comprising a stepof tempering the steel sheet at a temperature of not lower than 400° C.and not higher than A_(c1) point for a period of time of not shorterthan 10 minutes and shorter than 2 hours, the continuous annealingprocess or the plating process comprising a step of holding the steelsheet at a temperature of not lower than A₁ point and not higher than A₃point for 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in said temperature range for 1 second or more.
 18. Amethod of producing the high strength steel sheet described in claim 2wherein the base phase structure is tempered martensite or temperedbainite, the method comprising a hot rolling process, a cold rollingprocess, a first continuous annealing process, a tempering process, anda second continuous annealing process or a plating process, the firstcontinuous annealing process comprising a step of holding a resultingsteel sheet in a heated state at a temperature of not lower than A₃point and a step of cooling the steel sheet to a temperature of nothigher than Ms point or a temperature of not lower than Ms point and nothigher than Bs point at an average cooling rate of not lower than 20°C./s, the tempering process comprising a step of tempering the steelsheet at a temperature of not lower than 400° C. and not higher thanA_(c1) point for a period of time of not shorter than 10 minutes andshorter than 2 hours, the second continuous annealing process or theplating process comprising a step of holding the steel sheet in a heatedstate at a temperature of not lower than A₁ point and not higher than A₃point for 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in said temperature range for 1 second or more.
 19. Amethod of producing the high strength steel sheet described in claim 2wherein the base phase structure comprises tempered martensite andferrite or comprises tempered bainite and ferrite, the method comprisinga hot rolling process, a tempering process, and a continuous annealingprocess or a plating process, the hot rolling process comprising a stepof terminating finish rolling at a temperature of not lower than(A_(r3)−50)° C. and a step of cooling a resulting steel sheet to atemperature of not higher than Ms point or a temperature of not lowerthan Ms point and not higher than Bs point at an average cooling rate ofnot lower tan 10° C./s and winding up the steel sheet, the temperingprocess comprising a step of tempering the steel sheet at a temperatureof not lower than 400° C. and not higher than Ac1 point for a period oftime of not shorter than 10 minutes and shorter than 2 hours, thecontinuous annealing process or the plating process comprising a step ofholding the steel sheet in a heated state at a temperature of not lowerthan A₁ point and not higher than A₃ point for 10 to 600 seconds, a stepof cooling the steel sheet to a temperature of not lower than 300° C.and not higher than 480° C. at an average cooling rate of not lower than3° C./s, and a step of holding the steel sheet in said temperature rangefor 1 second or more.
 20. The method of claim 19, wherein the hotrolling process comprises a step of terminating finish rolling at atemperature of not lower than (A_(r3)−50)° C., a step of cooling thesteel sheet to a temperature in the range of 700±100° C. at an averagecooling rate of not lower than 30° C./s, a step of cooling the steelsheet with air in said temperate range for 1 to 30 seconds, and a stepof subsequently cooling the steel sheet to a temperature of not higherthan Ms point or a temperature of not lower than Ms point and not higherthan Bs point at an average cooling rate of not lower than 30° C./s andwinding up the steel sheet.
 21. The method of claim 19, wherein thecontinuous annealing process comprises a step of holding the steel sheetat a temperature of not lower than A₁ point and not higher than A₃ pointfor 10 to 600 seconds, a step of cooling the steel sheet to atemperature of (A₁ point to 600° C.) at an average cooling rate of nothigher than 15° C./s, a step of cooling the steel sheet to a temperatureof not lower than 300° C. and not higher than 480° C. at an averagecooling rate of not lower than 20° C./s, and a step of holding the steelsheet in said temperature range for 1 second or more.
 22. A method ofproducing the high strength steel described in claim 2 wherein the basephase structure comprises tempered martensite and ferrite or comprisestempered bainite and ferrite, the method comprising a hot rollingprocess, a cold rolling process, a first continuous annealing process, atempering process, and a second continuous annealing process or aplating process, the first continuous annealing process comprising astep of holding a resulting steel sheet at a temperature of not lowerthan A₁ point and not higher than A₃ point and a step of cooling thesteep sheet to a temperature of not higher than Ms point or atemperature of not lower than Ms point and not higher than Bs point atan average cooling rate of not lower than 10° C./s, the temperingprocess comprising a step of tempering the steel sheet at a temperatureof not lower than 400° C. and not higher than A_(c1) point for a periodof time of not shorter than 10 minutes and shorter than 2 hours, thesecond continuous annealing process or the plating process comprising astep of holding the steel sheet at a temperature of not lower than A₁point and not higher than A₃ point for 10 to 600 seconds, a step ofcooling the steel sheet to a temperature of not lower than 300° C. andnot higher than 480° C. at an average cooling rate of not lower than 3°C./s, and a step of holding the steel sheet in said temperature rangefor 1 second or more.
 23. The method of claim 22, wherein the secondcontinuous annealing process comprises a step of holding the steel sheetat a temperature of not lower than A₁ point and not higher than A₃ pointfor 10 to 600 seconds, a step of cooling the steel sheet to atemperature of (A₁ point to 600° C.) at an average cooling rate of nothigher than 15° C./s, a step of cooling the steel sheet to a temperatureof not lower than 300° C. and not higher than 480° C. at an averagecooling rate of not lower than 20° C./s, and a step of holding the steelsheet in said temperature range for 1 second or more.
 24. A method ofproducing the high strength steel sheet described in claim 3 wherein thebase phase structure is tempered martensite or tempered bainite, themethod comprising a hot rolling process and a continuous annealingprocess or a plating process, the hot rolling process comprising a stepof controlling a heat treatment temperature before hot rolling to atemperature in the range of 950° to 1100° C., a step of terminatingfinish rolling at a temperature of not lower than (A_(r3)−50)° C., and astep of cooling a resulting steel sheet to a temperature of not higherthan Ms point or a temperature of not lower than Ms point and not higherthan Bs point at an average cooling rate of not lower than 20° C./s andwinding up the steel sheet, the continuous annealing process or theplating process comprising a step of holding the steel sheet in a heatedstate at a temperature of not lower than A₁ point and not higher than A₃point for 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in said temperature range for 1 second or more.
 25. Amethod of producing the high strength steel sheet described in claim 3wherein the base phase structure is tempered martensite or temperedbainite, the method comprising a hot rolling process, a cold rollingprocess, a first annealing process, and a second annealing process or aplating process, the hot rolling process comprising a step ofcontrolling a heat treatment temperature before hot rolling to atemperature in the range of 950° to 1100° C., the first continuousannealing process comprising a step of holding a resulting steel sheetin a heated state at a temperature of not lower than A₃ point and a stepof cooling the steel sheet to a temperature of not higher than Ms pointor a temperature of not lower than Ms point and not higher than Bs pointat an average cooling rate of not lower than 20° C./s, the secondcontinuous annealing step or the plating step comprising a step ofholding the steel sheet at a temperature of not lower than A₁ point andnot higher than A₃ point for 10 to 600 seconds, a step cooling the steelsheet to a temperature of not lower than 300° C. and not higher than480° C. at an average cooling rate of not lower than 3° C./s, and a stepof holding the steel sheet in said temperature range for 1 second ormore.
 26. A method of producing the high strength steel sheet describedin claim 3 wherein the base phase structure comprises temperedmartensite and ferrite or comprises tempered bainite and ferrite, themethod comprising a hot rolling process and a continuous annealingprocess or a plating process, the hot rolling process comprising a stepof controlling a heat treatment temperature before hot rolling to atemperature in the range of 950° to 1100° C., a step of terminatingfinish rolling at a temperature of not lower than (A_(r3)−50)° C., and astep of cooling the steel sheet to a temperature of not higher than Mspoint or a temperature of not lower than Ms point and not higher than Bspoint at an average cooling rate of not lower than 10° C./s and windingup the steel sheet, the continuous annealing process or the platingprocess comprising a step of holding the steel sheet in a heated stateat a temperature of not lower than A₁ point and not higher than A₃ pointfor 10 to 600 seconds, a step of cooling the steel sheet to atemperature of not lower than 300° C. and not higher than 480° C. at anaverage cooling rate of not lower than 3° C./s, and a step of holdingthe steel sheet in said temperature range for 1 second or more.
 27. Themethod of claim 26, wherein the hot rolling process comprises a step ofcontrolling a heat treatment temperature before hot rolling to atemperature in the range of 950° to 1100° C., a step of terminatingfinish rolling at a temperature of not lower than (A_(r3)−50)° C., astep of cooling the steel sheet to a temperature in the range of700±100° C. at an average cooling rate of 30° C./s, a step of coolingthe steel sheet with air in said temperature range for 1 to 30 seconds,anda step of subsequently cooling the steel sheet to a temperature ofnot higher than Ms point or a temperature of not lower than Ms point andnot higher than Bs point at an average cooling rate of not lower than30° C./s and winding up the steel sheet.
 28. The method of claim 26,wherein the continuous annealing process comprises a step of holding thesteel sheet in a heated state at a temperature of not lower than A₁point and not higher than A₃ point for 10 to 600 seconds, a step ofcooling the steel sheet to a temperature of (A₁ point to 600° C.) at anaverage cooling rate of not higher than 15° C./s, a step of cooling thesteel sheet to a temperature of not lower than 300° C. and not higherthan 480° C. at an average cooling rate of not lower than 20° C./s, anda step of holding the steel sheet in said temperature range for 1 secondor more.
 29. A method of producing the high strength sheet described inclaim 3 wherein the base phase structure comprises tempered martensiteand ferrite or comprises tempered bainite and ferrite, the methodcomprising a hot rolling process, a cold rolling process, a firstcontinuous annealing process, and a second continuous annealing processor a plating process, the hot rolling process comprising a step ofcontrolling a heat treatment temperature before hot rolling to atemperature in the range of 950° to 1100° C., the first continuousannealing process comprising a step of holding a resulting steel sheetin a heated state at a temperature of not lower than A₁ point and nothigher than A₃ point and a step of cooling the steel sheet to atemperature of not higher than Ms point or a temperature of not lowerthan Ms point and not higher than Bs point at an average cooling rate ofnot lower than 10° C./s, the second continuous annealing process or theplating process comprising a step of holding the steel sheet at atemperature of not lower than A₁ point and not higher than A₃ point for10 to 600 seconds, a step of cooling the steel sheet to a temperature ofnot lower than 300° C. and not higher than 480° C. at an average coolingrate of not lower than 3° C./s, and a step of holding the steel sheet insaid temperature range for 1 second or more.
 30. The method of claim 29,wherein the second continuous annealing process comprises a step ofholding the steel sheet in a heated state at a temperature of not lowerthan A₁ point and not higher than A₃ point for 10 to 600 seconds, a stepof cooling the steel sheet to a temperature of (A₁ point to 600° C.) atan average cooling rate of not higher than 15° C./s, a step of coolingthe steel sheet to a temperature of not lower than 300° C. and nothigher than 480° C. at an average cooling rate of not lower than 20°C./s, and a step of holding the steel sheet in said temperature rangefor 1 second or more.